Reportedly, Minnesota Gov. Tim Walz, the Democratic nominee for vice president, is nervous about the VP debate tonight. But really, the one who should be nervous is Republican nominee Sen. J.D. Vance (R-OH).
There is little evidence that running mates in general have much of an impact on the election, although this election is very close so anything might matter. Dick Cheney's strong debate performances in 2000 and 2004, and Pence's strong performance against Tim Kaine in 2016 might have contributed to the very narrow victories their parties achieved, although that's more anecdotal than analytical. The worst vice-presidential debate performance was probably that of Dan Quayle, running mate to George H.W. Bush, in 1988. When Quayle, who was perceived as callow and too young, stated that he had the same level of experience as JFK did when he became president, his Democratic opponent, Sen. Lloyd Bentsen, pounced, stating, "Senator, I served with Jack Kennedy. I knew Jack Kennedy. Jack Kennedy was a friend of mine. Senator, you're no Jack Kennedy."
But Bush-Quayle went on to win the election.