The V.P. Debate Might Boost Vance and Walz, but It Probably Won’t Shift the Polls

When Senator JD Vance of Ohio and Gov. Tim Walz of Minnesota took the debate stage Tuesday night, a significant share of Americans had not yet formed opinions about either vice-presidential candidate, according to recent polls.

Many voters will now have a better sense of who they are, other than just the running mates of former President Donald J. Trump and Vice President Kamala Harris. But history suggests the debate is unlikely to sway voting choices.

Since vice-presidential debates have typically been followed by presidential debates a few days later, it has been hard to measure their impact on past elections. 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 538.

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). More recently, snap polls after the vice-presidential debate in 2020 between Harris and Mike Pence 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.