Part of the reason Mrs. Clinton’s bounce seems more likely to last is that it seems to be coming from greater party unity: growing support from Bernie Sanders’s backers. The Marist poll showed Mrs. Clinton with the support of 90 percent of his voters, while a CNN poll gave her 91 percent. These numbers had usually been in the 60s or 70s.

Mrs. Clinton’s support appears to be underpinned by her improving image among Democratic-leaning voters, according to Gallup data. She could still lose some of those voters to third-party candidates, but Mr. Trump faces similar risks.

These gains have allowed Mrs. Clinton to move up to around 47 or 48 percent of the vote in recent polls. This is not an especially impressive tally, but for that same reason it seems reasonable to expect she could keep it: She’s not winning the support of a huge number of voters whom you would expect to abandon her.

At the same time, the unusually large number of voters considering a third-party candidate make Mrs. Clinton’s tallies seem likelier to be enough to win.