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.
Clinton currently has 483 super delegates in her corner, according to CNN's count, a massive lead over Sanders' 31.
Sanders' campaign has started a two-pronged attack on Clinton's super delegate lead: Sanders is publicly knocking the super delegate system, and suggesting that delegates from states he won should flip to his corner, while his campaign is urging delegates one-by-one to back the Vermont senator.
Clinton's campaign has said they are not worried about losing super delegates.
"There is no indication that these folks have flipped," Joel Benenson, Clinton's senior strategist said, noting that if delegates were flipping, the Sanders campaign would be touting it.
The Sanders campaign has also used county and state conventions in caucus states to sway delegates in a complicated process that could net them a handful of pledged delegates. By being more organized and diligent, the Vermont senator hopes to shrink Clinton's lead.
Most recently the Sanders campaign won a share of delegates at Nevada country conventions that outpaced the actual number of delegates they won during the February caucus, a development that Sanders' campaign manager Jeff Weaver said could net the campaign upward of eight delegates.