We’re not big fans of national polls here at FiveThirtyEight. In the general election, they provide less information than state polls do, especially given that the presidency is determined by the Electoral College. But at least in the general election, everybody votes on the same day.1 Not so in the primaries, where the states vote sequentially. Furthermore, the rules vary substantially from state to state; in particular, some hold primaries and others have caucuses, which generally have much lower turnout. That makes it difficult to determine what a “likely voter” is in the context of a national poll.

So we’re not quite sure how much to read into national polls that show the Democratic race having tightened substantially. Our own national polling average has Hillary Clinton ahead of Bernie Sanders by 7.6 percentage points, essentially2 her narrowest margin of the campaign. That lead is down from 18.5 percentage points on Jan. 31, the day before Iowa voted.

Other polling averages show an even tighter race, with Clinton up by just a couple of percentage points. We could spend some time debating the “right” way to calculate a national polling average — given that there’s no national primary, our method is designed to be deliberate rather than rush to place a lot of weight on new polls. But no matter whose numbers you’re looking at, Sanders has gained on Clinton.

If Sanders has gained on Clinton, however, shouldn’t we also see evidence of that from the states that have voted so far? Other things held equal, we’d expect him to perform better in states voting in April than those voting in March, and better in March than in February.
http://fivethirtyeight.com/features/more-democrats-are-feeling-the-bern-probably/