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Bernie Sanders said on Sunday that he and Hillary Clinton were heading to a “contested” convention this summer because she will need superdelegates to secure the nomination, a claim that clashes with the accepted definition of a contested convention. He also said that superdelegates who have supported her should switch to him instead.

At a news conference in Washington, Mr. Sanders said that the Democratic convention in July would be contested because “it is virtually impossible for Secretary Clinton to reach the majority of convention delegates by June 14 with pledged delegates alone,” and that she “will need superdelegates to take her over the top.” He added: “In other words, the convention will be a contested convention.”

A convention is typically understood as contested when a candidate cannot reach the necessary delegate count using both pledged delegates and superdelegates, typically party officials and state leaders who are not bound to any candidate.

Mr. Sanders urged superdelegates in states that he has won and those who came out in support of Mrs. Clinton before he declared his candidacy to switch their support to him. He also said other superdelegates should consider supporting him because in many polls he beats Donald J. Trump by more than points Mrs. Clinton does, and that they would be more likely to do so if he won further primaries.

He acknowledged that his strategy to persuade the superdelegates to switch would be difficult.

“We have an uphill climb, no questions about it,” he said. “It is not going to be easy. Nothing I’m telling you today suggests that this is going to be an easy fight.”

Mr. Sanders’s push for superdelegates comes as he fights to remain viable after a string of losses last week and a sharp drop in fundraising in April. Mr. Sanders has the support of 39 superdelegates, while 520 have said they will support Mrs. Clinton. She also leads him in pledged delegates, 1,663 to 1,367, giving her a total lead of 2,183 to 1,406. It takes 2,383 total delegates to secure the nomination.

Mr. Sanders expressed frustration that Mrs. Clinton had won superdelegates even in states where he won the primary. In Washington State, where he won almost 73 percent of the vote, Mrs. Clinton has 10 superdelegates while he has none. In Colorado, Mr. Sanders won 59 percent of the vote, but again Mrs. Clinton has 10 superdelegates from that state and he has none. Sanders aides handed out a list showing similar situations in states like New Hampshire, Kansas and Maine where he won more votes but has fewer superdelegates than his rival.

“If I win a state with 70 percent of the vote, you know what? I think I am entitled to those superdelegates,” Mr. Sanders said. “I think the superdelegates should reflect what the people of the state want, and that’s true for Hillary Clinton as well.”

According to Democratic Party rules, pledged delegates must follow the wishes of voters while superdelegates can support whichever candidate they want. In 2008, Barack Obama won the party’s nomination against Mrs. Clinton using pledged and superdelegates.

Still, Mr. Sanders was adamant about his strategy. He called Mrs. Clinton the party’s “second strongest candidate,” and said he hoped superdelegates would focus on his strength among independents and young people.

“I think the real struggle will be in the hearts and the minds of superdelegates who came onboard the Clinton campaign a long, long time ago,” Mr. Sanders said. “That is admittedly a tough road to climb, but it is not an impossible road to climb, and we intend to fight for every vote in front of us and for every delegate remaining.”