What’s consistent is the contention that debates are more often than not a minor factor, eclipsed by others. “They tend to confirm existing trends,” wrote Karl Rove, the longtime strategist for George W. Bush, in The Wall Street Journal last week. Looking at the Trump-Clinton matchup, he added: “With so many unhappy voters, a big mistake on either side could scramble the contest. But don’t count on it.”

That’s reassuring, but only slightly. The “unhappy voters” he refers to make 2016 especially volatile. Beyond the large pool of undecided Americans, there are Clinton supporters who don’t really like her that much and Trump supporters with a gnawing recognition of his ridiculousness. More than 10 percent of Americans support a third-party contender, and some of these voters could return to the Democratic or Republican fold in the weeks to come.

Trump may well have an edge in wooing them, because the share of Americans who believe that the country is on the wrong track is more than double the share who believe that it’s on the right track. They’re desperate for a swerve. Clinton isn’t the most plausible agent of that, not after a quarter-century as a Democratic standard-bearer, not with a previous stint in the White House, not with Obama imploring Americans to honor and safeguard his legacy by choosing her.