Vermont Sen. Bernie Sanders is waging a campaign at a turning point. The New York primary earlier this week was essentially a must win. And he lost.

He's still campaigning as hard as ever, hopping from state to state talking about a rigged economy and a political system ruled by millionaires and billionaires. But the candidate who started out as an underdog and rose to heights few expected has a math problem.

Sanders needs to win all the remaining contests by a 20-point margin to catch up to Clinton in pledged delegates.

"There's a pathway to victory for Sen. Sanders, but I think you know it's certainly gotten brambled," said Neil Sroka with the progressive group Democracy for America, which supports Sanders.

Hillary Clinton's lead over Sanders in pledged delegates is more than twice as large as then-Sen. Obama ever had in 2008. Even so, Sroka hopes Sanders will stay in it through the final contests in June, running a spirited campaign.

"There is not a single doubt in my mind that the strong campaign that Bernie Sanders is waging right now is making the Democratic Party better, stronger and more focused on the populist progressive issues that we need to take on if we are going to be successful in November," Sroka said.

But unless something dramatic changes, Sanders and his campaign are going to face a moment of reckoning. Mo Elleithee is executive director of the Georgetown Institute of Politics and Public Service, and, in 2008, he worked on the Clinton campaign.

"We were behind," Elleithee said. "We had a late burst of momentum. But the math was never there for us."

Eventually, a little less than a month before the final primary, Clinton began holding her fire. The campaign made an emotional and tactical shift. Elleithee says there are two ways to go — graciously or lashing out.

"It's a dangerous place to be in," Elleithee said, "and you've got to keep your head about you and remember what it is you're fighting for. That doesn't mean you have to get out, but it does mean you need to kind of keep your focus in the right place."

On Tuesday, five more states hold primaries, and polling suggests Clinton is in for another good night. When asked if there would come a point when Sanders tones down his criticisms of the front-runner, as Clinton did late in the 2008 race, Sanders senior campaign adviser Tad Devine said he'd see what happens on Tuesday.

"If we think we've made enough progress, then we'll keep on the path that we're on," Devine said. "If we think we have to, you know, take a different way or reevaluate, you know, we'll do it then. But right now, we think the best path beyond is the one we laid out months ago."

That path involves winning a lot in the weeks ahead — and finally overtaking Clinton in pledged delegates in June. Although some in Sanders' orbit are talking about a fight at the convention in July, Devine said the only winning strategy is for Sanders to come out ahead in pledged delegates. And that won't be easy.

Right now, the Clinton campaign and other Democrats are getting worried that some of Sanders' attacks are going to leave a mark that will hurt her in the general election, should she be the nominee. Sanders raised significantly more money than Clinton in March and has been able to outspend her on television ads in state after state. To maintain her lead, Clinton has to keep campaigning hard, running ads and not fully turning her attention to the general-election fight.

Elleithee said the way Sanders and his surrogates handle the next month-and-a-half will determine his campaign's legacy.

"People ought to feel good about what he did," Elleithee said. "The problem is too often campaigns in this position end up squandering a lot of that goodwill as part of the end game."

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