Sen. Bernard Sanders‘ campaign began laying the groundwork Tuesday for a contested convention this summer, saying Hillary Clinton won’t be able to win the nomination based on the primaries and the two will end up fighting for the superdelegates who will ultimately make the decision.

Sanders campaign manager Jeff Weaver said they will try to persuade the hundreds of superdelegates who are backing Mrs. Clinton to ditch her and swing behind Mr. Sanders before the July convention in Philadelphia.

“If you look at the math … it is very, very, very unlikely either candidate, either Secretary Clinton or Sen. Sanders, will go into the convention with a majority needed of pledged delegates in order to win,” Mr. Weaver told CNN. “And so, in essence, the Democratic convention will be an open convention. [The superdelegates] don’t count until they vote, and they don’t vote until they get to the convention. So, when we arrive at the convention, it will be an open convention.”

Heading into the Wisconsin primary Tuesday, Mrs. Clinton had 1,712 delegates to Mr. Sanders‘ 1,011, according to an Associated Press tally. Among pledged delegates, Mrs. Clinton leads Mr. Sanders 1,243 to 980.

Mrs. Clinton holds a massive lead among superdelegates, 469 to 31. It takes 2,383 delegates to secure the nomination.

There is little evidence that superdelegates are willing to switch their support from Mrs. Clinton to Mr. Sanders despite increasing pressure from liberal activists.

In Alaska, Democratic committeewoman and superdelegate Kim Metcalfe has pledged to back Mrs. Clinton, even though Mr. Sanders won more than 80 percent of the vote in the state’s caucuses last month.

On the other side, Alaska Democratic Party Vice Chairman Larry Murakami said last week that he would back Mr. Sanders after the senator’s huge win in the state.

Mrs. Clinton’s camp says the superdelegate fight is premature because she is leading in the basic voting, winning more states and taking a significantly greater share of the popular vote than Mr. Sanders.

“For most of the campaign, Senator Sanders has criticized the role that superdelegates play in the nominating process, but as he now campaigns without a clear path to the nomination that relies on the voters, he’s aggressively courting their support,” Clinton campaign manager Robby Mook wrote in a memo.

Mr. Sanders is already behind schedule if he is to win over some of Mrs. Clinton’s superdelegates. In 2008, Barack Obama began to earn his first switchers at the beginning of the year, a signal that Mrs. Clinton’s campaign was in trouble.

So far, there is no evidence of a similar swell of defections, and Mr. Sanders hasn’t won many superdelegates on his own.

While snatching the nomination from Mrs. Clinton at this year’s convention seems to be a long shot at best, political analysts say, there is also little reason for Mr. Sanders to exit the race. He continues to win states — including a sweep of Washington, Hawaii and Alaska on March 26 — and is raising much more money than Mrs. Clinton.

In the process, he has excited progressive voters in a way that Mrs. Clinton simply has not, said Eric Kasper, a political science professor at the University of Wisconsin-Eau Claire.

“Hillary Clinton has done what she needs to do on many fronts, as she has raised a significant amount of money, secured support among numerous superdelegates, obtained many key endorsements and put in place a solid paid and volunteer staff operation. However, Sanders has an outsider message on economics that appeals to a sizable share of progressives, and he is seen as more authentic by many people,” Mr. Kasper said.

Mrs. Clinton said she was pleased to see Mr. Sanders injecting so much enthusiasm into the Democratic ranks but expressed confidence that she would be the nominee.

“I’m really excited that he’s attracted so many people, particularly young people, into this contest,” she said during an appearance on “The View.” “And, again, you have to look at the broader perspective. We’ve won some. He’s won some. But I have 2½ million more votes than he does. And I have a very significant lead in delegates.”

Moving forward, both campaigns will turn their full attention to New York. Just a month ago, Mrs. Clinton held a lead of more than 20 points in the state she represented in the U.S. Senate for eight years, but that gap has since closed.

The most recent Real Clear Politics average of New York polls has Mrs. Clinton ahead by 11 points, and Mr. Sanders believes he has a real shot to win the state and make a serious dent in the former first lady’s delegate advantage.

“We’re going to be doing the best throughout the city of New York, throughout upstate New York, and we’re feeling pretty good,” Mr. Sanders told NBC News on Tuesday.

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