Story highlights Hillary Clinton's campaign says its lead over Bernie Sanders' organization is insurmountable

The Vermont senator hopes to pick off super delegates

Cohoes, New York (CNN) Hillary Clinton's campaign manager wrote in a memo published Monday that the former secretary of state's presidential campaign has a near "insurmountable lead" in delegates and that the only way for rival Bernie Sanders to win the nomination would be to reverse "the will of the voters."

Months ago Clinton's campaign did not expect to be fighting this hard ahead of the April 19 New York primary. Even so, by writing the memo, campaign manager Robby Mook is arguing Sanders is spinning his wheels by staying in the race.

"The math being what it is, the Sanders campaign has struggled to explain their path to the nomination," Mook wrote. "Their latest strategy involves a combination of trying to flip pledged delegates at state and county conventions, while also convincing super delegates that he deserves their support -- despite the fact that Hillary Clinton has won 58% of the popular vote and a majority of pledged delegates thus far."

Mook continued: "For most of the campaign, Senator Sanders has criticized the role that super delegates play in the nominating process, but as he now campaigns without a clear path the nomination that relies on the voters, he's aggressively courting their support."

Super delegates -- Democratic officials and elected officials who count as one delegate in the Democratic nomination process -- are not required to follow the will of their state. So if Florida votes a certain way, their members of Congress -- all of whom are super delegates -- are not required to follow the popular vote.

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