Recent general-election polling has generally shown Donald Trump maintaining a slight lead over President Biden. Yet many of those polls also reveal an Achilles’ heel for Mr. Trump that has the potential to change the shape of the race.

It relates to Mr. Trump’s legal troubles: If he is criminally convicted by a jury of his peers, voters say they are likely to punish him for it.

A trial on criminal charges is not guaranteed, and if there is a trial, neither is a conviction. But if Mr. Trump is tried and convicted, a mountain of public opinion data suggests voters would turn away from the former president.

Still likely to be completed before Election Day remains Special Counsel Jack Smith’s federal prosecution of Mr. Trump for his alleged scheme to overturn the 2020 election, which had been set for trial on March 4, 2024. That date has been put on hold pending appellate review of the trial court’s rejection of Mr. Trump‘s presidential immunity. On Friday, the Supreme Court declined Mr. Smith’s request for immediate review of the question, but the appeal is still headed to the high court on a rocket docket. That is because the D.C. Circuit will hear oral argument on Jan. 9 and likely issue a decision within days of that, setting up a prompt return to the Supreme Court. Moreover, with three other criminal cases also set for trial in 2024, it is entirely possible that Mr. Trump will have at least one criminal conviction before November 2024.

The negative impact of conviction has emerged in polling as a consistent through line over the past six months nationally and in key states. We are not aware of a poll that offers evidence to the contrary. The swing in this data away from Mr. Trump varies — but in a close election, as 2024 promises to be, any movement can be decisive.

To be clear, we should always be cautious of polls this early in the race posing hypothetical questions, about conviction or anything else. Voters can know only what they think they will think about something that has yet to happen.

Yet we have seen the effect in several national surveys, like a recent Wall Street Journal poll. In a hypothetical matchup between Mr. Trump and Mr. Biden, Mr. Trump leads by four percentage points. But if Mr. Trump is convicted, there is a five-point swing, putting Mr. Biden ahead, 47 percent to 46 percent.

In another new poll by Yahoo News-YouGov, the swing is seven points. In a December New York Times-Siena College poll, almost a third of Republican primary voters believe that Mr. Trump shouldn’t be the party’s nominee if he is convicted even after winning the primary.

The damage to Mr. Trump is even more pronounced when we look at an important subgroup: swing-state voters. In recent CNN polls from Michigan and Georgia, Mr. Trump holds solid leads. The polls don’t report head-to-head numbers if Mr. Trump is convicted, but if he is, 46 percent of voters in Michigan and 47 percent in Georgia agree that he should be disqualified from the presidency.

It makes sense that the effect is likely greater in swing states: Those are often places where a greater number of conflicted — and therefore persuadable — voters reside. An October Times/Siena poll shows that voters in the battleground states of Arizona, Georgia, Michigan, Nevada and Pennsylvania favored Mr. Trump, with President Biden narrowly winning Wisconsin. But if Mr. Trump is convicted and sentenced, Mr. Biden would win each of these states, according to the poll. In fact, the poll found the race in these six states would seismically shift in the aggregate: a 14-point swing, with Mr. Biden winning by 10 rather than losing by four percentage points.

The same poll also provides insights into the effect a Trump conviction would have on independent and young voters, which are both pivotal demographics. Independents now go for Mr. Trump, 45 percent to 44 percent. However, if he is convicted, 53 percent of them choose Mr. Biden, and only 32 percent Mr. Trump.

The movement for voters aged 18 to 29 was even greater. Mr. Biden holds a slight edge, 47 percent to 46 percent, in the poll. But after a potential conviction, Mr. Biden holds a commanding lead, 63 percent to 31 percent.

Other swing-state polls have matched these findings. In a recent survey in The Atlanta Journal-Constitution, for example, 64 percent said that they would not vote for a candidate whom a jury has convicted of a felony.

National polls also offer accounts of potential unease. In a Yahoo News poll from July, 62 percent of respondents say that if Mr. Trump is convicted, he should not serve as president again. A December Reuters-Ipsos national poll produced similar results, with 59 percent of voters overall and 31 percent of Republicans saying that they would not vote for him if he were convicted.

New data from our work with the Research Collaborative confirm the repercussions of a possible conviction on voters. These questions did not ask directly how a conviction would affect people’s votes, but they still support movement in the same direction. This survey, conducted in August and repeated in September (and then repeated a second time in September by different pollsters), asked how voters felt about prison time in the event that Mr. Trump is convicted. At least two-thirds (including half of Republicans) favored significant prison time for Mr. Trump.

Why do the polls register a sharp decline for Mr. Trump if he is convicted? Our analysis — including focus groups we have conducted and viewed — shows that Americans care about our freedoms, especially the freedom to cast our votes, have them counted and ensure that the will of the voters prevails. They are leery of entrusting the Oval Office to someone who abused his power by engaging in a criminal conspiracy to deny or take away those freedoms.

We first saw this connection emerge in our testing about the Jan. 6 hearings; criminality moves voters significantly against Mr. Trump and MAGA Republicans.

But voters also understand that crime must be proven. They recognize that in our legal system there is a difference between allegations and proof and between an individual who is merely accused and one who is found guilty by a jury of his peers. Because so many Americans are familiar with and have served in the jury system, it still holds sway as a system with integrity.

Moreover, recent electoral history suggests that merely having Mr. Trump on trial will alter how voters see the importance of voting in the first place. In the wake of the Jan. 6 committee hearings, the 2022 midterms saw turnout at record levels in states where at least one high-profile MAGA Republican was running.

The criminal cases are also unfolding within a wider context of other legal challenges against Mr. Trump, and they may amplify the effect. That includes several state cases that seek to disqualify him under Section 3 of the 14th Amendment. Colorado’s top court has already ruled that he is disqualified, though the case is now likely being appealed to the Supreme Court. This constellation of developments — also encompassing the New York civil fraud trial — offer a negative lens through which Americans may view Mr. Trump.

Again, this is all hypothetical, but the polls give us sufficient data to conclude that felony criminal convictions, especially for attacking democracy, will foreground the threat that Mr. Trump poses to our nation and influence voters in an election-defining way.

Norman Eisen was special counsel to the House Judiciary Committee for the first impeachment and trial of Donald Trump. Celinda Lake is a Democratic Party strategist and was a lead pollster for Joe Biden’s 2020 presidential campaign. Anat Shenker-Osorio is a political researcher, campaign adviser and host of the “Words to Win By” podcast.

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