Former president Donald Trump made an unusual foray into political science in July, arguing that “historically, the vice-president, in terms of the election, does not have any impact.” At the time, his running mate J.D. Vance was stumbling through a gaffe-filled rollout, having been drafted two weeks earlier. It seemed as if Mr Trump’s academic pronouncement was only intended to comfort himself—but it may have some merit. Today the theory will be put to the test, with Mr Vance facing Minnesota’s governor, Tim Walz, in their first and only televised debate of the election.