There's no debate that Clinton's poll numbers are improving

Usually, a good way to test the durability of a swing in the polls is to ask whether it brings the race closer or farther from the so-called fundamentals, like the president’s approval ratings.

A shift that brings the polls in line with the fundamentals might be a little likelier to last than one that cuts the other way. Mr. Kerry’s recovery, for instance, brought the tighter race implied by Mr. Bush’s approval ratings. Barack Obama’s gains in 2008 gave him the considerable advantage implied by the economy and Mr. Bush’s low approval ratings. The opposite could be said of Mr. Bush’s and Mr. Romney’s surges in 2000 and 2012.

This year, it’s a lot harder to tell. That’s in part because there is no incumbent president on the ballot, which always makes it a little harder to tell where the natural resting point of a race sits. But it’s also because Mr. Trump is such an extraordinary candidate that many analysts believe the fundamentals will be less significant than usual.

In lieu of the traditional fundamentals, here’s something to consider instead: Over the longer term, Mrs. Clinton has led Mr. Trump by around five percentage points nationally.