Former President Donald J. Trump at a rally in Anchorage last weekend.Credit…Ash Adams for The New York Times

As Donald J. Trump weighs whether to open an unusually early White House campaign, a New York Times/Siena College poll shows that his post-presidential quest to consolidate his support within the Republican Party has instead left him weakened, with nearly half the party’s primary voters seeking someone different for president in 2024 and a significant number vowing to abandon him if he wins the nomination.

By focusing on political payback inside his party instead of tending to wounds opened by his alarming attempts to cling to power after his 2020 defeat, Mr. Trump appears to have only deepened fault lines among Republicans during his yearlong revenge tour. A clear majority of primary voters under 35 years old, 64 percent, as well as 65 percent of those with at least a college degree — a leading indicator of political preferences inside the donor class — told pollsters they would vote against Mr. Trump in a presidential primary.

Mr. Trump’s conduct on Jan. 6, 2021, appears to have contributed to the decline in his standing, including among a small but important segment of Republicans who could form the base of his opposition in a potential primary contest. While 75 percent of primary voters said Mr. Trump was “just exercising his right to contest the election,” nearly one in five said he “went so far that he threatened American democracy.”

Overall, Mr. Trump maintains his primacy in the party: In a hypothetical matchup against five other potential Republican presidential rivals, 49 percent of primary voters said they would support him for a third nomination.

The greatest threat to usurp Mr. Trump within the party is Gov. Ron DeSantis of Florida, who was the second choice with 25 percent and the only other contender with double-digit support. Among primary voters, Mr. DeSantis was the top choice of younger Republicans, those with a college degree and those who said they voted for President Biden in 2020.

While about one-fourth of Republicans said they didn’t know enough to have an opinion about Mr. DeSantis, he was well-liked by those who did. Among those who voted for Mr. Trump in 2020, 44 percent said they had a very favorable opinion of Mr. DeSantis — similar to the 46 percent who said the same about Mr. Trump.

Should Mr. DeSantis and Mr. Trump face off in a primary, the poll suggested that support from Fox News could prove crucial: Mr. Trump held a 62 percent to 26 percent advantage over Mr. DeSantis among Fox News viewers, while the gap between the two Floridians was 16 points closer among Republicans who mainly receive their news from another source.

The survey suggests that Mr. Trump would not necessarily enter a primary with an insurmountable advantage over rivals like Mr. DeSantis. His share of the Republican primary electorate is less than Hillary Clinton’s among Democrats was at the outset of the 2016 race, when she was viewed as the inevitable front-runner, but ultimately found herself embroiled in a protracted primary against Senator Bernie Sanders of Vermont.

Mr. Trump’s troubles inside his party leave him hamstrung in a matchup against an unusually vulnerable incumbent.

The Times/Siena poll suggested that the fears of many Republican elites about a Trump candidacy may be well-founded: He trailed President Biden, 44 percent to 41 percent, in a hypothetical rematch of the 2020 contest, despite plummeting support for Mr. Biden, with voters nationwide giving him a perilously low 33 percent job-approval rating.

A growing anyone-but-Trump vote inside the party contributed to Mr. Trump’s deficit, with 16 percent of Republicans saying that if he were the nominee they would support Mr. Biden, would back a third-party candidate, wouldn’t vote at all or remained unsure what they would do. That compared to 8 percent of Democrats who said they would similarly abandon Mr. Biden in a matchup with Mr. Trump.

For Mr. Trump, bleeding that amount of Republican support would represent a sharp increase compared with the already troubling level of the party’s vote he shed during his last race.

In 2020, 9 percent of Republicans voted for someone other than Mr. Trump, while Mr. Biden lost just 4 percent of Democrats, according to AP VoteCast, a large study of the 2020 electorate by NORC at the University of Chicago for The Associated Press.

Kenneth Abreu, a 62-year-old pharmaceutical executive from Pennsylvania, said he had voted Republican for three decades but would support Mr. Biden instead of voting again for Mr. Trump.

“Unlike all these other people who believe every word he says, I’m done,” Mr. Abreu said. “All the garbage he’s been talking about, the lies, Jan. 6, the whole thing — I just lost all respect for him.”

Still, many Republicans who favor someone else in a primary would nonetheless rally behind Mr. Trump if he won the nomination.

Richard Bechtol, a 31-year-old Republican voter in Columbus, Ohio, said he would back either Mr. DeSantis or Senator Ted Cruz of Texas over the former president. Mr. Bechtol was disturbed by Mr. Trump’s behavior that led to the Capitol riot on Jan. 6, 2021.

“I hope he doesn’t run at all,” Mr. Bechtol said of the former president.

Mr. Bechtol, a lawyer, said he found Mr. Trump’s arrogance off-putting, saw Mr. Trump as a divisive figure in the party and believed that he bore responsibility for the violence.

But he said he would support Mr. Trump in 2024 in a rematch with President Biden.

“Biden is getting bullied by the left wing of his party and I worry about his cognitive function as well — actually, I worry about that with Trump, too,” he said. “It’s really a lesser-of-two-evils situation for me.”

It is too early to tell whether the challenges for Mr. Trump inside his party will result in anything more than speed bumps on his path to the Republican nomination. Underscoring his residual strength, he is viewed favorably by 65 percent of Republicans who said they would vote against him in a primary, compared with 33 percent who said they had an unfavorable view.

“Trump did a hell of a job on the economy,” said Marie Boyce, a New York Republican in her 70s. “There isn’t anything wrong I could say about him.”

David Beard, a 69-year-old retiree in Liberal, Mo., who said he mostly relied on Social Security for his income, said he was frustrated with both political parties and all levels of government. He plans to stick with Mr. Trump in 2024, betting that was the best chance to improve the economy.

“When Trump was in office, it didn’t seem like prices went haywire,” Mr. Beard said.

He said Democrats’ efforts to hold Mr. Trump accountable for the Jan. 6 attack had been a pointless distraction. “The government’s whole focus should have been on the people of the United States and the situation we’re in, instead of wasting time and money trying to impeach him,” Mr. Beard said. “Nothing is being done to help the people, and I believe that with all my heart.”

About 20 percent of all registered voters said they didn’t like either Mr. Trump or Mr. Biden. Mr. Trump also trailed his successor among these voters, 39 percent to 18 percent. One in five volunteered to pollsters that they would sit out such an election, though that option had not been offered to them.

“I never thought I would say this, but it if was Biden and Trump I don’t think I would vote,” said Gretchen Aultman, a 74-year-old retired lawyer in Colorado who voted for Mr. Trump in 2016. “I liked Trump’s policies, but he was so abrasive and unpolished, and having him as president was just tearing the country apart.”

Ms. Aultman said she didn’t see the current president as an acceptable alternative. “I can’t in good conscience vote for Biden,” she said. “I recognize the signs of being old, and his mental acuity is not going to last another two years.”

Between the large number of primary voters ready for another nominee, and the growing number who say they would not vote for the former president again under any circumstances, the poll suggests Mr. Trump’s biggest hurdle to winning a second term isn’t another Republican opponent — it’s himself.

John Heaphy, a 70-year-old retired software engineer in Arizona, said he voted for Mr. Trump in 2020 but planned to back Mr. Biden in 2024 because of the Capitol riot.

Mr. Heaphy said that Mr. Trump had incited an insurrection, and that he was shaken by the support the former president’s false claims have received from other Republicans. Indeed, according to the poll, 86 percent of Republicans who said they would support Mr. Trump in the 2024 primary said he was the legitimate winner of the 2020 election.

“Trump lost the election,” Mr. Heaphy said. “There are too many people out there that just don’t seem like they believe in reality anymore.”

While Mr. Trump has described election integrity as the country’s most pressing concern, just 3 percent of Republicans named it as the nation’s top problem. But Mr. Trump’s response to his 2020 defeat was a significant factor in how Republicans are thinking about 2024.

Among Republicans who said they plan to vote against Mr. Trump in a primary, 32 percent said the former president’s actions threatened American democracy.

Paula Hudnall, a 51-year-old nurse in Charleston, W.Va., said Mr. Trump was right to question the results of the election. She said she didn’t blame him for the violence at the Capitol.

“Anytime you have a large gathering you’re going to have people who get out of hand and are unruly,” said Ms. Hudnall, who identified the economy and infrastructure as her top issues.

Ms. Hudnall said she was interested in learning about other Republican candidates, but that Mr. Trump already had her vote again for 2024.

The Times/Siena survey of 849 registered voters nationwide was conducted by telephone using live operators from July 5 to 7. The margin of sampling error is plus or minus 4 percentage points. Cross-tabs and methodology are available here.

Isabella Grullón Paz and Nate Cohn contributed reporting.

Credit…Kenny Holston for The New York Times

The tenacious work of the Jan. 6 committee has transformed how we think about the Jan. 6 rebellion. It should also transform the Justice Department’s investigation into efforts to overturn the 2020 election.

Before the hearings, federal agents and prosecutors were performing a classic “bottom up” criminal investigation of the Jan. 6 rioters, which means prosecuting the lowest-ranking members of a conspiracy, flipping people as it proceeds and following the evidence as high as it goes. It was what I did at the Justice Department for investigations of the Genovese and Colombo crime families, Enron and Volkswagen as well as for my part in the investigation of Russia’s interference in the 2016 election led by the special counsel Robert Mueller.

But that is actually the wrong approach for investigating the Jan. 6 insurrection. That approach sees the attack on the Capitol as a single event — an isolated riot, separate from other efforts by Donald Trump and his allies to overturn the election.

The hearings should inspire the Justice Department to rethink its approach: A myopic focus on the Jan. 6 riot is not the way to proceed if you are trying to follow the facts where they lead and to hold people “at any level” criminally accountable, as Attorney General Merrick Garland promised.

The evidence gathered in the hearings describes a multiprong conspiracy — what prosecutors term a hub and spoke conspiracy — in which the Ellipse speech by President Trump and the Jan. 6 attack on the Capitol were just one “spoke” of a grander scheme.

This broader approach would avoid the thorny debate that has emerged as to whether Mr. Trump could be criminally culpable for inciting the riot during his Ellipse speech or if, on the contrary, his speech is protected under the First Amendment and the evidence too ambiguous to justify the extraordinary step of indicting a former president. Building a criminal case that looks solely at the riot itself is far more complex legally and factually for those who weren’t at or in the Capitol. These challenges of the current bottom-up approach have led to criticism of the slow pace of the narrow Justice Department approach.

Instead, what the hearings have revealed is evidence of a plot orchestrated by Mr. Trump and his allies in the White House and elsewhere — including players from the Mueller investigation like Roger Stone, Michael Flynn and Rudy Giuliani as well as new players like Jeffrey Clark and John Eastman. The “spoke” of the Jan. 6 riot should be seen and investigated simultaneously with the other “spokes”: orchestrating fake electors in key states, pressuring state officials like those in Georgia to find new votes, plotting to behead the leadership of the Justice Department to promote a lackey who would further the conspiracy by announcing a spurious investigation into election fraud, and pressuring Vice President Mike Pence to violate the law.

Investigating the Jan. 6 insurrection in the context of the other means by which Mr. Trump appears to have sought to undermine the transfer of power serves to strengthen any future case by presenting the complete evidence of the perpetrators’ actions and intent. And it undermines possible defenses.

For instance, the evidence that Mr. Trump lied in a statement about Mr. Pence’s agreeing that he had the power to reject electors undermines the defense that Mr. Trump was acting in good faith and honestly believed he had won the election. And Mr. Trump’s conduct in the White House after his speech at the Ellipse and during the Jan. 6 attack, which includes remarks, reported from testimony by the former White House aide Cassidy Hutchinson, in which Mr. Trump condoned the chants calling for hanging of the vice president of the United States, is strong evidence of his intent for a plan to upend a democratic election.

There are signs that the department, spurred on by the committee, has begun to look into some of these other “spokes.” Unsurprisingly, the “spoke” involving the Justice Department itself has attracted acute interest. Recently, federal agents conducted a search of the home of Mr. Clark, whom Mr. Trump considered elevating to be acting head of the Justice Department, and seized the phone of Mr. Eastman, a lawyer who advised Mr. Trump on efforts to overturn the election.

But other signs are not so encouraging: Department prosecutors were reportedly surprised by the testimony of Ms. Hutchinson. That is not a sign of a robust investigation into the facts. The department has more tools than Congress does to learn the truth. It could have interviewed Ms. Hutchinson long ago, as well as many others whose evidence is relevant — indeed, Ms. Hutchinson alone provided investigators numerous leads to pursue.

For those who do not voluntarily cooperate with a Justice Department investigation, prosecutors can serve grand jury subpoenas and obtain their testimony under oath, subject to criminal penalties like perjury — just as Georgia state prosecutors are doing. And people can be given immunity to compel their testimony if they validly assert the Fifth Amendment. Obtaining grand-jury testimony is indispensable; it forestalls witnesses from credibly claiming later that they had not made certain statements in an interview or that an interview report is inaccurate (or worse).

I have been involved in numerous high-profile investigations that engendered significant congressional interest, and what I have seen in this inquiry is not typical behavior from the Justice Department. Usually, department prosecutors and agents don’t want Congress jumping ahead of their investigation, and they work hard to make sure that doesn’t happen. The department wants to interview witnesses first, and prosecutors make sure that targets are fully truthful about their own potential wrongdoing and that their testimony is corroborated; use tools to flip recalcitrant witnesses; and build a case without revealing evidence to other prospective witnesses — efforts that can falter if Congress is conducting private and public interviews that may inadvertently undermine the strongest possible criminal case.

Department lawyers and congressional committees usually work collegially to avoid these issues, something that can happen when Congress has faith in the diligence and resolve of the Justice Department. That does not appear to be happening here. We have seen the unusual public filing of a letter from Justice Department leadership seeking access to committee evidence, something largely unnecessary if it had already obtained that evidence. And the fact that the letter was sent is a clear sign of a breakdown in the relationship between the two branches, something I did not see even during high-profile investigations like Enron and the Mueller investigation.

The American public is entitled to a thorough, fearless, competent and fair criminal investigation. That is still possible, and what facts that investigation reveals, and what prosecutorial decisions are made thereafter, will surely be subject to debate. But until we pursue all leads, that debate will be truly academic, to the detriment of our democracy.

Andrew Weissmann (@AWeissmann_), a former Justice Department prosecutor and senior prosecutor in Robert Mueller’s special counsel investigation, is a professor of practice at the New York University School of Law and the author of “Where Law Ends: Inside the Mueller Investigation.”