Can a Vice-Presidential Debate Shift the Polls? History Shows It’s Unlikely.

A significant share of Americans have not formed opinions about Senator JD Vance of Ohio and Gov. Tim Walz of Minnesota. That means Tuesday’s vice-presidential debate between the two men may help some voters develop a better sense of who they are, other than just the names of the running mates of former President Donald J. Trump and Vice President Kamala Harris.

But while past vice-presidential debates have delivered some memorable moments, there is little evidence that they have a meaningful impact on elections overall.

Since vice-presidential debates are typically followed by presidential debates a few days later, it is difficult to measure their impact. Over the last six presidential election cycles, there was only one time when national polls moved more than one percentage point after a vice-presidential debate and before the next presidential debate, according to an analysis by FiveThirtyEight.

That was in 2000, and even then, it was only a 1.2-point change (toward George W. Bush, after his running mate, Dick Cheney, sparred with Joseph I. Lieberman, Al Gore’s vice-presidential pick).

The debate on Tuesday is the last scheduled debate of this election, so it could make more of an impression. But it also may not move the needle much. Consider the vice-presidential debate in 2020 between Ms. Harris and Mike Pence: Snap polls found that respondents’ likelihood of voting for either presidential candidate was identical before and after the debate.

Research has also shown that even outside of debates, vice-presidential candidates themselves have little effect on voters’ choices.

What these debates can do is give voters more familiarity with the running mates — for better or worse.

Polls show that about one in five Americans have