After months of voicing doubt about a challenge from the vice president, Clinton campaign operatives are viewing Mr. Biden’s entry into the contest as a serious possibility and are trying to rally the party’s apparatus and its donors to her side. They have flooded uncommitted Democrats with emails, phone calls and a plea for them to sign a letter, a copy of which was obtained by The New York Times. In the letter, Democrats are asked to “pledge to support Hillary Rodham Clinton at the 2016 Democratic National Committee Convention with my unpledged delegate vote.”

A campaign aide said that given the mistakes of Mrs. Clinton’s 2008 campaign, they had always planned to make an early and aggressive push to lock down superdelegate support…

“I think they’re trying to create the impression that it will be difficult for anybody to climb up and beat Hillary in the superdelegate count,” said Ed Rendell, a former governor of Pennsylvania who is supporting Mrs. Clinton. But, he added, “I wouldn’t pressure people too hard if I were them.”