This theory helps explain why the most abrupt movements in the polls seem to come when the race is near one of its two extremes. Mrs. Clinton surged after the first presidential debate in part because she was near her low point heading into it. The allegations about Mr. Trump’s sexual misconduct didn’t hurt him as much in the polls as his first debate performance, for instance, because Mrs. Clinton’s marginal supporters had already flocked back to her column after the debate. There were fewer voters for her to lure back.

With this history in mind, it would make sense if the newest F.B.I. revelations took a modest toll on Mrs. Clinton’s standing in the polls. Mr. Trump’s position has improved over the last week or so, but he still trailed in national polls by about five or six points before Friday, when news of Mr. Comey’s letter to Congress about the new emails surfaced. There’s room for Mrs. Clinton’s standing to fall a bit, or perhaps for Mr. Trump to continue to consolidate Republican voters.

But the same history makes it harder to argue that a change in the polls would represent a lasting shift in the race.

If most of the movement in the polls can be attributed to varying degrees of enthusiasm among voters at the periphery of each candidate’s coalition, then the polls might move without any real change in the underlying race at all.

To be clear, this is not to say that Mr. Trump can’t win, or that shifts in the polls can’t reflect a significant underlying shift in voter sentiment. It’s possible that variations in enthusiasm really do correlate, for instance, with the willingness of voters to turn out.

One can imagine the perfect storm: Democratic-leaning voters, newly dispirited by the news about Mrs. Clinton’s emails, decide not to turn out; well-educated, Republican-leaning white voters return to their traditional party after a few weeks of relative quiet from Mr. Trump and bad news about Mrs. Clinton; undecided, Democratic-leaning white working-class men break for Mr. Trump; Hispanic and black turnout falls short of expectations, and so on.

But so far this cycle, it has been safer to bet that the polls will return to where they’ve been than to assume that a new shift will be a lasting one. It points toward a clear, fundamental dynamic in the race: A majority of voters dislike both candidates and they would prefer not to vote for either one of them, but they believe Mr. Trump is not fit to be president.