Of those who switched to Mrs. Clinton, 80 percent did not have a four-year college degree. Only 65 percent of those who stayed with one of her rivals were in this category. With the exception of men, those moving to Mrs. Clinton look a lot like the set of people who initially supported her.

Other cross-sectional polls have reported Mrs. Clinton’s losses among white Democratic women. The YouGov data show something more nuanced — that the probability of moving to and from Mrs. Clinton at this point differs between men and women. This gender dynamic seems to have little to do with age, education, income, marital status or interest in public affairs. In other words, it’s happening in addition to the effects from these other things, not because gender is correlated with these characteristics.

One possibility is that women were supporting Mrs. Clinton after her announcement in higher-than-usual numbers because of the prospect of electing the first woman to the White House. Once the excitement of her entrance into the race wore off, some decline among women seems logical. But this doesn’t explain the concomitant gains among men.

The debate phase allows all of the declared Democratic candidates to speak to primary and caucus voters nationwide. In coming months, Mrs. Clinton will be trying to bring women and white voters back to her side, while her rivals will be hoping to attract men and minorities.

There has been much less volatility on the Democratic side so far this year than among likely Republican primary voters. Some of this may be because there are fewer candidates in the Democratic race. But more likely the stability is coming from the fact that Mrs. Clinton entered this race a strong front-runner and remains a formidable candidate — even after a summer that finished weaker than it started.