The CNN poll showed that working-class supporters of Mrs. Clinton were likelier to say that they were supporting her as a vote against Mr. Trump, not because they supported her. Between those figures and Mrs. Clinton’s weak overall number with the group, she does not appear to have built her lead with voters who might be especially likely to return to Mr. Trump.

The polls now put Mrs. Clinton a bit ahead of the level she held before James Comey, the F.B.I. director, excoriated her for her use of a private email server. She held a similar lead in April — before Mr. Trump won the nomination in May and narrowed her lead for the month or so that followed. This has been the race’s natural resting place when there hasn’t been something else suppressing Mrs. Clinton’s support — another reason to think she might be able to retain most of her bounce.

For Mr. Trump, the danger is that he has few opportunities to prevent Mrs. Clinton from further consolidating her support over the coming weeks. There are few reasons to think he has helped himself since the convention by firing back at the parents of Humayun Khan and drawing condemnation from some Republican leaders.

August is typically a fairly quiet month in presidential politics. The Olympics soak up national media coverage, which could make it harder for Mr. Trump to break through. Mrs. Clinton’s extensive paid advertising will continue unabated. Perhaps the most famous August shift in recent memory came from the “Swift Boat” ads that eroded John Kerry’s standing after his convention in 2004.

If Mrs. Clinton retains most of her gains over the coming weeks, Mr. Trump’s chances in the race will start to look fairly bleak. Surveys conducted a few weeks after the conventions are far more predictive of the result than those taken ahead of or during the conventions.

No modern presidential candidate who trailed in the polls a few weeks after the conventions has gone on to win the popular vote.