Some of Bernie Sanders's most die-hard supporters have begun saying that they'll never vote for Hillary Clinton in a general election — even if that gives Republicans control of the White House.

Don't count on them following through on the threat.

Rallying around the slogan "Bernie or Bust," Sanders loyalists say they'll sit out a general election or support Green Party candidate Jill Stein if Sanders isn't the Democratic nominee — a prospect that's becoming increasingly likely as the delegate math continues to tip in Clinton's favor.

In one poll, 33 percent of Sanders of voters say they won't vote for Clinton, and more than 60,000 people have already signed a pledge to that effect.

Is there reason to believe the "Bernie or Bust" movement is actually serious? Of course, anything could happen — but here's why some experts express doubt that Sanders's supporters won't come around to Clinton eventually.

Why it's likely Sanders loyalists will come around to Clinton

Of course, it's theoretically possible that a big enough core of Sanders's devotees will refuse to back Clinton to tip the balance of a general election.

But two experts — Alan Abramowitz, a political science professor at Emory University, and Matt Grossmann, director of the Institute for Public Policy and Social Research at Michigan State University — think it's exceedingly unlikely.

Here are some of the key reasons the political scientists gave:

They mostly agree on big policy questions: Sanders and Clinton may seem at odds right now, but they stand an ocean apart from the major Republican candidates on an overwhelming number of positions. "The differences between Sanders and Clinton on policy just aren't very great, and they get blown out of proportion in a primary contest," Abramowitz said. There are far too many to list here, but a sample include: the need to raise taxes on the wealthy, the reality of climate change, appointing a progressive to the Supreme Court, the desire not to deport millions of undocumented immigrants, and raising the minimum wage.

A (relatively) civil primary: Sanders and Clinton have traded some barbs over their past voting records, and Sanders has knocked her for taking exorbitant speaking fees from Wall Street. But the primary hasn't been that acrimonious. "On our worst days ... we are 100 times better than any Republican candidate," Sanders said in February. There have been few attacks driven by the candidates' personalities or personal histories. "The race hasn't been very bitter; much less bitter than that between (Barack) Obama and Clinton, for instance," Grossman said.

Sanders himself: Sanders himself has pledged to back Clinton if he loses the primary, reiterating in March that both candidates' key goal must be to not allow the Republicans to win back the White House.



.@BernieSanders: "Hillary Clinton and I agree that it is absolutely imperative that no Republican make it to the Oval Office." — Sahil Kapur (@sahilkapur) March 26, 2016 This effect will be even more powerful if Sanders gives a major speech at the Democratic National Convention urging his supporters to rally behind Clinton, Abramowitz noted.

Historical precedent: In July 2008, 54 percent of Clinton voters said they wouldn't support Barack Obama in a general election. (They even had a nickname, "PUMAs" — "party unity my ass," the 2008 analog to today's "Bernie or Busters.") Ultimately, however, nine in 10 Democrats ended up voting for Obama over John McCain, according to the Nation. Similar threats were made — and later failed to materialize — from the supporters of Howard Dean in 2004, according to Abramowitz. "Just about every time there's a closely contested nomination battle, the supporters of the candidate who appear to be losing threaten to walk out," Abramowitz said. "Then they don't."

The specter of a Trump or Cruz presidency: This historical precedent is coupled with the strong likelihood that the Republicans will nominate Donald Trump or Ted Cruz, who are both widely reviled on the left. "Clinton's been leading Trump by an average margin of 10 points, and that wouldn't be the case if a significant number of Democrats were defecting," Abramowitz said.

What are the reasons to believe this conventional wisdom is wrong?

Is there any reason to believe that something will be fundamentally different this year?

One argument advanced by Sanders's supporters is that we're in an "anti-establishment" year — in which both primary electorates are ignoring cues from party leaders — and that the independents who have flocked to the Vermont senator are therefore unlikely to support Clinton, the consummate insider.

Another possible variable is that young people, who have thrilled to Sanders in huge numbers, will feel less of an allegiance to stay within and support the Democratic Party nominee. That makes their support for the nominee less likely to be a sure bet.

"Young people are certainly less tied to the political parties in general," Grossman says. "They're less likely to have stable partisan voting patterns, and that makes them more likely to be open to alternatives."

An additional possibility that could scramble this analysis: if Sanders pulls off an improbable upset and somehow manages to win the pledged delegate race, but then loses the nomination because the superdelegates throw the race to Clinton.

That defiance of the voters really could lead to a backlash of Sanders loyalists against the party, Abramowitz said.

"That could be a problem — then you would see Sanders's supporters, and maybe Sanders himself" refusing to support Clinton, Abramowitz said.

Democrats' support for Obama should help Clinton unify the party

There is one scenario in which the supporters of a losing primary candidate tend to really not support the party's nominee: when an unpopular presidential incumbent faces a tough challenger.

"When you see an incumbent president seeing a serious challenge for the nomination, that's always a bad sign," Abramowitz said, noting that primary challenges against Gerald Ford in 1976 and Jimmy Carter in 1980 both reflected their weakness in the general election.

I asked Abramowitz if this year could prove another iteration of this basic dynamic. After all, isn't Clinton largely running for Obama's third term?

He rejected that notion out of hand. Yes, Clinton has run under the banner of Obama's third term — but that should help, not hurt, her efforts to unify the party, he said.

"Obama is actually very popular among Democrats," Abramowitz said, noting the president has a nearly 90 percent approval rating among members of his party. "If there was more discontent among rank-and-file voters with Obama, there would be more opportunity for an anti-establishment candidate in the general."