Why Hillary Clinton’s Polling Bounce Has a Better Chance to Persist

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What a month. At the end of a series of tumultuous events and two political conventions, the presidential race is more or less where it was before it all began: Hillary Clinton has a clear lead.

All seven national surveys conducted since the Democratic convention show her ahead, by an average of nearly seven percentage points.

It’s a seven-point boost over where those same surveys showed the race after the Republican convention — enough to erase Donald Trump’s bounce and more. She is about three points ahead of where she was before the two conventions.

Convention bounces often prove to be short-lived, as the name implies and as Mr. Trump’s experience proves. Often, post-convention bounces don’t even reflect real changes in support — just changes in how likely people are to respond to a poll or to indicate their support for a candidate.

With that history in mind, Mrs. Clinton’s lead would be expected to fade a bit over the next few weeks. But her comfortable advantage in the post-convention polls suggests that her support can fade and that she can still maintain a clear lead.

There are also reasons to think that Mrs. Clinton’s bounce is likelier to stick than most. In general, convention bounces are most durable when they help unify the parties — something that’s largely inevitable and hard to undo.
The latest CNN poll, which showed Mrs. Clinton ahead by nine points, suggests that she made her biggest gains among friendly groups, like young voters, supporters of Bernie Sanders and nonwhite voters. These gains could prove relatively durable. Notably, these gains have been enough to push Mrs. Clinton to 50 percent in three of the post-convention surveys.

Mrs. Clinton made gains among Republicans and white voters without a college degree, according to the CNN poll. But she’s still faring worse among those voters than the CNN poll has shown in the past, and even worse among white working-class voters than President Obama fared in pre-election polls from 2012.

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 popu.lar vote.