National polls show the race tightening -- but state polls don't

Among the six polls that had previously surveyed the same state, Clinton gained ground in every one, with an average swing of 6 percentage points toward her. A caution: The average shift is inflated by a Quinnipiac poll of Colorado which found Clinton up 8 points; Quinnipiac had implausibly showed an 11-point lead for Trump when it surveyed the race in November. Even without that poll, however, Clinton’s average gain is 4 percentage points, still pretty good.

There are a couple of further nuances that explain some of the differences. Most of the recent national polls are daily or weekly tracking polls conducted online or via automated surveys, and these tracking polls have generally been a relatively friendly group for Trump. He hasn’t fared well recently in traditional telephone surveys, by contrast, with one or two exceptions like his not-so-bad result in the Pew Research poll. Also, looking at the trend lines doesn’t quite make for an apples-to-apples comparison, because most of the national polls have surveyed the race multiple times since the conventions, while the state polls haven’t. It’s plausible that Clinton is polling slightly off her post-convention peak, as the national polls suggest, but ahead of where she was for most of the pre-convention period, as the state polls suggest.