Sen. Bernie Sanders (I-Vt.) (Photo by Jabin Botsford/The Washington Post)

THE MORNING PLUM:

Has Sanders tacitly joined the “Bernie or Bust” movement? Could he somehow try to imperil Hillary Clinton’s chances in a general election by doing less than what is necessary to try to swing his national following — particularly young voters — behind Clinton, if she wins the nomination?

Nah. I just can’t see it.

In an interview with me today, senior Sanders adviser Tad Devine left no doubt: Not only will Bernie Sanders support Clinton if she is the nominee; he will also do everything possible to make sure the next president is a Democrat, even if it isn’t Bernie Sanders.

“Bernie has said he’s going to support the nominee, and I’m sure he’ll do everything to make sure that the next president is a Democrat,” Devine told me. “We believe Bernie will be the nominee, and we hope Clinton will give us the same kind of vigorous endorsement.”

“But that doesn’t mean he wouldn’t be energetic and vigorous and work as hard as he possibly could” for the Dem nominee if it is Clinton, Devine said. “He worked hard for Obama twice.”

“Bernie understands that having someone like Trump or Cruz become president of the United States would be destructive to the future of this nation,” Devine continued. “There’s no way that he’s going to be in any way associated with anything that allows that to occur.”

Devine said, however, that “we think the best way to make sure that doesn’t happen is to have the strongest Democratic nominee,” pointing out that some polls have shown Sanders stronger in a general election against GOP candidates than Clinton is. Devine added that having a Dem nominee with Clinton’s negatives could be “catastrophic.”

All of this comes after the New York Times reported that Sanders’s suggestion that Clinton is “not qualified” for the presidency “sent a shudder through party officials aligned with Mrs. Clinton.” Meanwhile, the Post reported that there is “widespread concern among Democrats that their rivalry is doing lasting damage to the party and the eventual nominee.”

Bernie Sanders said Hillary Clinton is not "qualified" to be president because of the donations she accepts from the fossil fuel industry, while Clinton questioned whether Sanders's ideas are realistic. (Peter Stevenson/The Washington Post)

And Paul Krugman’s column today tore into Sanders’s “not qualified” claim, arguing that Sanders’s case rests on imposing a ridiculous purity test on Clinton, asking: “Is Mr. Sanders positioning himself to join the ‘Bernie or bust’ crowd, walking away if he can’t pull off an extraordinary upset, and possibly helping put Donald Trump or Ted Cruz in the White House?”

I doubt it. For one thing, there are Devine’s comments above. For another, leading Democratic figures such as Elizabeth Warren (who has carefully maintained a form of neutrality that perfectly positions her to play a peacekeeping role later) may be able to prevail on Sanders supporters to get behind the nominee.

And finally, the most compelling reason Sanders probably will do the right thing and work to unite the party is this: Climate change. Just yesterday, Sanders tweeted: “Climate change is the single greatest threat facing our planet.” He also tweeted:

Sanders cannot undermine the Dem nominee if he really believes these things, which he surely does. As Sanders knows, even if it’s true that his climate agenda is more ambitious than Clinton’s is, she would do everything possible to facilitate U.S. engagement in the Paris global climate accord and make it a success — including protect Obama’s Clean Power Plan, which is key to our ability to meet our international commitments — while a GOP president would do everything possible to undermine both.

While Sanders has said the global deal doesn’t go far enough to address the problem, he has also said it is a “step forward,” while suggesting that Republicans don’t “care about the future of the planet.” Given all this, it’s very hard to see how he could possibly do anything that could be perceived as even minimally helpful to GOP efforts to capture the White House — and roll back the climate progress that is now being made at a critical moment.

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* BERNIE CLARIFIES THAT HILLARY IS ‘QUALIFIED’: Sanders cleaned up the mess in an interview with NBC’s Savannah Guthrie:

“Here’s the truth,” Sanders said. “I’ve known Hillary Clinton for 25 years. I respect Hillary Clinton. We were colleagues in the Senate, and on her worst day, she would be an infinitely better president than either of the Republican candidates.”

“She’s qualified?” Guthrie asked. “Of course,” Sanders said, “but the point is I would hope we get away from these attacks, which by the way, the media likes very much, and start focusing — maybe we can do that today — on the real issues.”

There you go!

* HILLARY LEADS IN CALIFORNIA, BUT BERNIE IS GAINING: A new Field poll finds that Hillary Clinton leads Bernie Sanders by 47-41 among likely Democratic primary voters in California. Details:

She remains the choice of women, older voters and registered Democrats here. But Sanders, the senator from Vermont, is pummeling Clinton by 25 percentage points among likely voters in their 30s, and by an even wider margin among younger voters….Sanders leads Clinton by 10 percentage points among independent voters likely to vote in the Democratic primary.

This is tighter than the double digit lead Clinton had in the last Field poll. But most of the other remaining primaries are registered Dems only. In California, which votes on June 7th, Sanders might have to win by as much as 15 points to help close the delegate gap.

* HILLARY LEADS IN NEW YORK, BUT BERNIE IS GAINING: Meanwhile, a new Emerson College poll being released today finds that Clinton leads Sanders by 18 points among likely Dem voters in New York by 56-38.

Again, that’s a substantial tightening, but that’s because Clinton led by nearly 50 points in the last Emerson poll. FiveThirtyEight’s polling average has her up 16 in New York. But Sanders might have to win the state by four points to help close the delegate gap.

* SANDERS AIDES ACKNOWLEDGE MATHEMATICAL REALITY: They privately acknowledge it, anyway:

Before crowds of thousands, Mr. Sanders has effusively played up his momentum, but privately his aides acknowledge a daunting mathematical reality: He must play catch-up in New York. Mrs. Clinton leads by roughly 250 pledged delegates, and a map that had been friendly to Mr. Sanders will shift back in her favor with primaries in New York, followed by Pennsylvania on April 26.

And remember, many of these primaries to come are closed to independents; Clinton has done far better in closed primaries than Sanders has.

* GOP INSIDERS EXPECT A CONTESTED CONVENTION: Politico talks to Republican strategists, operatives, and activists in 10 battleground states, and concludes:

Republican insiders overwhelmingly believe this summer’s national convention will require multiple ballots to select the presidential nominee….with roughly 90 percent of respondents saying neither Donald Trump nor Ted Cruz will win the nomination on the first ballot in Cleveland….Some insiders gave Trump, who needs to win just under 60 percent of the remaining delegates to clinch the nomination outright, an outside chance to win on the first ballot – but only if he over-performs in the upcoming states.

Meanwhile, Nate Silver calculates that Trump will need to get at least 40 percent of the vote in upcoming contests to clinch a majority of delegates and the nomination, which he hasn’t been able to do thus far.

* HERE’S WHO COULD DECIDE TRUMP’S FATE: Philip Rucker has a detailed overview of an important group to watch: the uncommitted delegates in each state who could help decide Trump’s fate at the convention:

These are the swing voters of the GOP nominating contest, nearly 200 activists and elected leaders beholden to nothing except their personal judgment and empowered to make or break candidacies. If Trump arrives at the July convention in Cleveland just shy of the 1,237 delegates required to secure the nomination outright, these unbound delegates could decide to push him over the top — or force a contested convention with successive rounds of balloting.

Navigating this will be a piece of cake for the vaunted author of “The Art of the Deal,” naturally.

 * AND TRUMP STAFFS UP FOR CONVENTION BATTLE: The Times reports that Trump has elevated Paul Manafort, a “veteran of floor fights,” to the position of piloting the campaign through the coming contested convention craziness. Manafort notes, interestingly, that the campaign has passed into a new phase, in which “how you win and where you win does matter,” and “protecting what you won” is crucial.

In other words, Trump is dramatically ratcheting up efforts to prevent Ted Cruz from stealing delegates and to prepare for all manner of procedural shenanigans designed to deny him the nomination. Fun times ahead!