Mook said that Clinton has amassed a "nearly insurmountable" delegate lead and has won more than 58 percent of the popular vote in the Democratic primary, making her the winner of more delegates and actual votes than Sanders.

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"The math being what it is, the Sanders campaign has struggled to explain their path to the nomination," Mook said. ". . . 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."

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The memo comes at a time when frustration from Clinton and her aides is boiling over. A contentious back-and-forth about a future debate in New York has launched a war of campaign statements. And last week, at the end of a rally in New York, Clinton lashed out at a Greenpeace activist who asked about contributions to her campaign from oil- and gas-company employees.

"I am so sick of the Sanders campaign lying about me!" Clinton fired back.

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With Sanders poised to potentially win the Wisconsin primary on Tuesday, Clinton's campaign has emphasized that a victory there won't change the dynamics of the race much.

More than just winning, Sanders needs to dominate Clinton in most of the large primaries that remain to catch up to her delegate count, Mook said.

"In order to do so, Sanders has to win the four remaining delegate-rich primaries — New York, Pennsylvania, California, and New Jersey — with roughly 60 percent of the vote," he noted. "To put that in perspective: Sanders has thus far won only two primaries with that margin: Vermont and New Hampshire."