Her campaign officials and allies have been making hundreds of phone calls to lock in delegate support ahead of the Aug. 19-22 Chicago convention while also seeking to block any would-be challengers, Reuters was first to report on Sunday.
Locking in delegate support is essential to ensuring Harris replaces Biden on the Nov. 5 ballot as the party's nominee against Republican Donald Trump.
Harris needs the support of a simple majority, or an estimated 1,969 of the 3,936 Democratic delegates, to secure her nomination at the August convention. About one-quarter have pledged to back her so far, based on public announcements.
Biden has endorsed Harris as his successor, but he cannot force delegates to follow his lead.
"Biden's endorsement of her has been very powerful in getting donors to give money and in delegates ... to ultimately pledge for her," Democratic National Committee Finance Chair Chris Korge, a Florida delegate, told Reuters.
"With him endorsing the vice president, I think by the end of business on Wednesday she'll probably have enough pledged delegates to go over 50%," Korge said.
The southern U.S. state of Tennessee became the first to pledge its 70 delegates to the vice president, sources told Reuters on Sunday.
Two top campaign officials said Wednesday is the goal, while another source with direct knowledge of the matter said "yes, working toward that" when asked about Wednesday.
The vice president's office and the Harris campaign declined to comment.
Most of the Democratic delegates are committed to Biden after state primaries this year. After his announcement he was pulling out of the race, they are not bound by any law or party rules to back anyone.
The powerful rules committee of the Democratic National Convention, set to meet on Wednesday, is charting a virtual plan for selecting a new nominee before the convention.
Jaime Harrison, chairman of the DNC, said in a letter to delegates last week that a virtual vote would take place between Aug. 1 and Aug. 5, in order to have the nomination process completed by Aug. 7, the date by which Ohio law had required a nominee to be in place to make the state's ballot.
Ohio lawmakers subsequently pushed back the deadline to Sept. 1, but party officials said they hoped to beat the Aug. 7 deadline to avoid any legal risk in the state.
