Bernie Sanders has pledged to do "everything I can" to stop Donald Trump from winning the White House, including supporting Hillary Clinton after the primary.

What's less clear is how many of Sanders's die-hard supporters will do the same. And at last night's victory rally in California, thousands of them erupted into raucous cries of "Bernie or bust" — the movement of Sanders's voters promising never to vote for the nominee if it's Clinton.

Sanders didn't hush the chant, smiling and letting his supporters' cries grow. That led to some public hand-wringing among prominent Democratic officials:

Voice in large, raucous CA crowd shouts "Bernie or bust!" And @SenSanders says "to beat @realDonaldTrump we have to beat @HillaryClinton." — David Axelrod (@davidaxelrod) May 18, 2016

How much should this scene scare Democrats?

Whether this is a bad sign for Democrats in November remains to be seen. Polling has found that very few Sanders supporters are likely to vote for Trump — Clinton leads Trump among Sanders's voters by an 87 to 13 margin — and Sanders consistently hammers away at Trump on the stump.

Moreover, while about 25 percent of Sanders's supporters now say they won't vote for Clinton, we're still very early on in the primary. Around this time of year in 2008, about 50 percent of Clinton voters said they'd never back Barack Obama — but then nine in 10 Democrats supported Obama over John McCain.

But looking at the choices of Sanders fans as Clinton versus Trump risks overlooking two important alternatives: that they either stay home on Election Day or vote for Green Party candidate Jill Stein.

And as Bill Scher points out in Politico, not that many of Sanders's fans have to reject Clinton to seriously hurt her chances in a close race. In swing states like Colorado or New Hampshire that are also filled with Sanders loyalists, Stein may need to only peel a few percentage points away from Clinton to cost her the state:

Even if Sanders isn’t deliberately trying to replicate the electoral trauma inflicted by [Ralph] Nader in 2000—when he probably cost Al Gore the presidency—Bernie’s lingering presence in the Democratic primary threatens to produce a similar result in November: delegitimizing the eventual Democratic nominee in the eyes of the left and sending many critics, if not to Trump, then to the Green Party’s Jill Stein or the Libertarian Party’s Gary Johnson. In the first poll to assess the impact of third-party candidates, Public Policy Polling found last week that the inclusion of Stein and Johnson shaves 2 percentage points off Clinton’s lead over Trump. Conversely, the minor party duo loses a combined 2 points when Sanders is tested as the Democratic nominee, indicating that Sanders’ voters account for Clinton’s reduced standing. A couple points, a couple million voters, is no big deal to Clinton if she’s trouncing Trump. But if he makes it a race, Democrats may find their political post-traumatic stress disorder from 2000 flaring up.

Vox's Andrew Prokop makes the argument today that Sanders isn't working to burn the Democratic Party to the ground, and that he's instead more likely to be playing a long game to maximize his leverage ahead of the Democratic National Convention.

Prokop may be right that Sanders ultimately plans on trying to bring Democrats around Clinton at the convention. Just how many of his supporters will be willing to go along with that plan, however, may be a different question altogether.