Nicole Gaudiano

USA TODAY

WASHINGTON — Sen. Bernie Sanders' decisive defeat in Tuesday’s Democratic presidential primary in New York apparently hasn't dampened his determination to campaign all the way to the party’s national convention in July.

“We believe we have the momentum and we believe we have a path toward victory,” Sanders said at a news conference in Vermont after Hillary Clinton declared the nomination race is in “the homestretch and victory is in sight.”

Sanders believes he can still win a majority of pledged delegates and then make the case to superdelegates — unpledged party leaders and elected officials who are free to support the candidate of their choice — that he's better positioned than Clinton to take on the Republican nominee.

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Sanders' campaign manager, Jeff Weaver, told MSNBC that the campaign will “absolutely” try at the convention to flip superdelegates committed to Clinton, even if Sanders remains behind in the popular vote and pledged delegates. And if Sanders defies expectations and ends up leading in pledged delegates, Clinton's superdelegates will shift their allegiance, said Tad Devine, a top Sanders adviser. He recalled that during the 2008 presidential nomination race, superdelegates who had committed to Clinton defected to Barack Obama.

“I think they will step back and they will say, ‘What’s the right thing to do for the party?," Devine said. "Should we be for somebody who was 50, 60 points ahead when this thing began and lost all of that lead to this guy? Or should we be for somebody who’s demonstrated in recent weeks that he’s by far the strongest candidate?’”

Clinton strategist Joel Benenson countered that Clinton’s New York win makes her lead “even more insurmountable.” He said Sanders’ superdelegate strategy isn’t realistic.

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“Why would (super)delegates, who Sen. Sanders has been highly critical of — not just for their role in the process but branding them as part of the establishment —why would they listen to that argument from him?” he said on MSNBC. “It’s just not an argument that savvy political people are going to buy.”

Sanders lapsed even further behind among pledged delegates after his 58%-42% loss in New York, a state Clinton represented in the Senate for eight years. At this point, Sanders would need to win 59% of remaining pledged delegates to claim a majority of those delegates. He would need to win more than 72% of all remaining delegates (including superdelegates) to claim the 2,383 needed to clinch the nomination.

After rallies in Pennsylvania Tuesday, Sanders returned home to Burlington, Vt., to get “recharged and take a day off.” He’ll return Thursday to Pennsylvania – among five states that will hold nominating contests next Tuesday – for a rally in Philadelphia and town halls in Scranton and Reading.

Clinton leads Sanders by an average 13 points in Pennsylvania, but Sanders' campaign believes he has a solid chance to win there and in other states next Tuesday. Sanders said he doesn’t plan to change his message about fighting income inequality and a corrupt campaign finance system because it's “resonating across the country.”

“I’ve always believed that it is grassroots activism that wins elections, and I think you’re going to see that in place next week,” he said.

The biggest problem for Sanders in the 19 remaining contests is that all but five -- Indiana, Montana, North Dakota, Puerto Rico and South Dakota -- are either closed or partially closed to independents, said Lara Brown, an associate professor at George Washington University’s graduate school of management. In partially closed contests, such as the one that will take place in delegate-rich California on June 7, independent voters typically must ask the Democratic Party for permission to vote.

“In these states where you have party machines and you have closed contests, it’s very hard for (Sanders') coalition -- which is really young people and independents -- to be sure that they’re on the rolls and able to cast their ballots,” Brown said.

Sanders’ coalition is less diverse than Clinton’s, which could hurt him in Maryland and California, but “his problem is more the process,” she said.

Sanders also faces challenges in winning over superdelegates who support Clinton and aren’t pleased with the nasty turn the Democratic campaign has taken.

One superdelegate, Democratic National Committee member Bob Mulholland from Chico, Calif., wrote a highly publicized letter alleging that Sanders' supporters have bullied superdelegates with “harassing emails, Facebook postings and phone calls,” demanding that they support the Vermont senator.

Mullholland said in an interview he was upset that Sanders’ campaign filed a complaint with the DNC on Monday accusing the the committee and Clinton of violating fundraising rules through use of a joint fundraising committee.

“They have zero chance of converting anyone from a Clinton supporter to a Sanders supporter,” he said.

Devine said the campaign’s outreach to superdelegates has been positive, and he disavowed any connection to harassing communications from others. In courting superdelegates, the campaign hopes to point to a lead in pledged delegates at the end of the primary season -- and to Sanders’ standing in national polls, he said.

Both Sanders and Clinton beat Republican candidates Donald Trump and Sen. Ted Cruz in hypothetical general-election matchups, but Sanders beats each of the two GOP candidates by a larger margin, according to a RealClearPolitics average of recent surveys. Sanders also is closing the gap with Clinton in national polls of Democratic voters.

“I think it’s a question of who’s going to win (in the general election),” Devine said. “That’s going to be the calculation” for super delegates.

In response to allegations that Sanders' campaign has turned negative, Devine noted that 46% of Democratic voters surveyed in New York exit polls said Clinton ran a more negative campaign, compared to 34% who said that of Sanders. Sixty-six percent said the Democratic campaign has energized the party, while 30% said it has divided it.

“This argument that’s being made that Bernie Sanders' presence is somehow detracting either from the energy of the campaign or that Bernie Sanders is the one running an unfair negative campaign is refuted, I think, pretty dramatically by what the voters are saying,” Devine said.