But despite big polling movement, Clinton has a gigantic and growing lead in another key metric with a strong track record of influencing the nomination process: Endorsements from elected Democrats.
A 56 percent majority of Democratic governors, senators and U.S. House members have endorsed Clinton for president, according to data tracked by FiveThirtyEight.
No other candidate has a significant share of support from elected Democrats. Joe Biden has 1.2 percent support for a potential run, while 0.4 percent (a.k.a. one person) goes to former Maryland governor Martin O'Malley. Sanders has no endorsements at all.
Clinton’s endorsement advantage represents a major barricade against Sanders’s run as well as a potential bid by Biden. Endorsements are a key indicator of candidates’ standing in the “invisible primary” -- the period before any caucuses and primaries in which candidates compete for the endorsement of party leaders, interest groups and activists. Endorsements provide a window into how the invisible primary is playing out, and most importantly, they have proven more predictive than early polls in how candidates eventually fare among voters in caucuses and primaries.
That’s one of the core conclusions that political scientists Marty Cohen, David Karol, Hans Noel and John Zaller found in their much-cited book, “The Party Decides.”
So how much does Clinton's endorsement lead actually matter? "I believe her endorsement lead is the best possible insulation against challenges to her front-runner status," said Cohen, an assistant professor at James Madison University. "When it comes to amassing delegates in the various state primaries and caucuses, those endorsements will provide her with the campaign infrastructure to turn out votes that should neutralize the efforts of her opponents."
Turning back to the endorsement data collected by FiveThirtyEight: Three components of the endorsement race demonstrate Clinton’s strength. What’s fascinating about the data above is not just Clinton’s advantage – it's big – but that she already has reached an absolute majority of endorsements among all potential endorsements. It's September -- five months before any actual voting. Even if Biden or Sanders rounded up every uncommitted Democratic senator, House member and governor, they would max out at 44 percent. Among elected Democrats who have endorsed anyone in the race so far, 97 percent have backed Clinton.
Secondly, Sanders hasn't gained a single endorsement. His campaign seems to acknowledge this as a weakness; The Post’s John Wagner reported last month that Sanders plans to make a concerted pitch for his candidacy to Democratic leaders. Sanders is of course running as an anti-establishment candidate, and this metric’s focus on support from elected leaders doesn't play to his strengths. Nonetheless, a lack of any establishment support signals the party -- as in, basically power within it -- is not rooting for his name on the ticket.
Thirdly, Clinton’s advantage is substantially larger than at this point in 2007, when Barack Obama was able to surge after winning the Iowa caucuses. Clinton held roughly one-third of all endorsement “points” tallied by FiveThirtyEight (giving governors and senators greater weight), while today she holds an outright majority. Her endorsement lead in 2008 was more vulnerable precisely because she had not locked up the public support of many Democrats by late 2007. This time around, most Democrats have already backed her.
Sanders's lack of endorsements warrants a caveat, and that is that endorsements of Democratic governors/senators/House members do not represent the views of all Democrats important in the invisible primary. Sanders likely performs better among liberal activists and other groups that also represent a key part of the party.
There's also the possibility that American politics has entered a new paradigm, in which traditional measures of campaign success like this are overblown or even irrelevant. Witness Donald Trump, who also has no establishment support but is still leading GOP primary race where the vast majority of Republican leaders have yet to endorse a candidate. Trump also has won no primaries or caucuses than Michelle Bachmann or Rick Perry.
Importantly, the authors of "The Party Decides" note that parties tend to seek a candidate who can appeal to broadly different parts of the party. Sanders's lack of officeholder endorsements indicates he does not yet have such broad appeal.
Clinton can only lose her lead in endorsements if elected leaders withdraw their previous support for her candidacy -- something elected leaders are loathe to do -- and do so en masse. But it’s not clear what problems with Clinton’s campaign would be serious enough for a leader to switch their endorsement.
While poll numbers may fluctuate up and down, Cohen notes there is little precedent for large-scale loss of endorsements by a presidential candidate. "I imagine if there is a smoking gun in the e-mail scandal, that could cause the foundation to crack and it could spread like it does on a cracked windshield," Cohen explained. "But absent that, I would not foresee endorsers abandoning her en masse, even for someone new like Joe Biden."
In short, Clinton would need to confront catastrophic problems to cause endorsers to flee, and that sort of bloodletting simply hasn't happened yet. Even with a loss in personal ratings this year, they were far from toxic in a Post-ABC poll last week showing 45 percent of Americans with a favorable impression and 53 percent unfavorable.
In other words, it will take much bigger changes to shift views of leading Democrats. And even after a frenetic summer, her endorsement lead has continued to grow.
Peyton M. Craighill contributed to this report.