In the hours since U.S. President Joe Biden bowed out of the presidential campaign, dozens of Democratic governors, donors, lawmakers and potential opponents came out swinging in support of Vice-President Kamala Harris as their would-be candidate.

But some of the party's most prominent heavyweights have held back — from Senators Chuck Schumer and Hakeem Jeffries to former U.S. president Barack Obama, who issued statements supporting Biden but stopped short of an endorsement for Harris. As of Monday night, Harris had secured the support of enough Democratic delegates to become her party's nominee against Republican Donald Trump, according to an Associated Press survey. 

"I have extraordinary confidence that the leaders of our party will be able to create a process from which an outstanding nominee emerges," Obama said in a statement Sunday, which did not mention Harris. 

Professors in American history and political science say some key Democratic leaders might be holding out on endorsements not because they doubt Harris or expect another challenger, but because they don't want to be seen as influencing the outcome of a race that's been upended so late in the game.

"I think they want to give the impression that the party gets the chance to make a decision without it being dictated by the elites of the party," said Paul Quirk, a professor of political science at the University of British Columbia. 

"I've seen no grounds for thinking that any of them actually would oppose her candidacy and, in particular, there hasn't been anyone else who seems likely to emerge."

Several state delegations met late Monday to confirm their support for Harris, including Texas and her home state of California. By Monday night, she had the support of at least 2,471 delegates, according to an AP tally, more than the 1,976 she'll need to win on a first ballot. No other candidate was named by a delegate contacted by AP.

Despite this, AP is not calling Harris the new presumptive nominee. That's because delegates are still free to vote for the candidate of their choice at the Democratic National Convention (DNC) in August, or if Democrats go through with a virtual roll call ahead of that gathering in Chicago.

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Kamala Harris lays out her case against Donald Trump as money, endorsements roll in

High-profile Democrats including Nancy Pelosi are backing U.S. Vice-President Kamala Harris for the Democratic nomination as the party racks up $81M in donations just one day after Joe Biden dropped out and endorsed Harris.

Harris intends to 'earn and win' spot on ticket

Schumer, the majority leader, and Jeffries, the minority leader, said in a joint statement that Harris was off to a "great start with her promise to pursue the presidential nomination in a manner consistent with the grassroots and transparent process set forth by the Democratic National Committee."

"Vice-President Kamala Harris has excited the community, she's excited the House Democratic caucus and she's exciting the country. I'm looking forward to sitting down with her, in person, in short order, with leader Schumer," Jeffries told reporters Monday, according to NBC's Meet The Press.

He said he'd have more to say about the path forward after that meeting.

Harris, 59, said in her own statement Sunday that she intends to "earn and win" a place at the top of the ticket.

Democrats including California Gov. Gavin Newsom, Michigan Gov. Gretchen Witmer and Transportation Secretary Pete Buttegieg — who were initially thought to be her most likely challengers — were quick to back Harris. 

Former House Speaker Nancy Pelosi, who is deeply influential among party ranks, joined the chorus of support later on Monday.

Party rules require that delegates vote on their leadership candidate, either virtually or at the DNC in Chicago on Aug. 19.

Typically, the conventions are a rubber-stamping process because delegates pledge their support for the candidate who wins the primaries. This year, that was Biden. 

WATCH | Democratic strategist on who Harris should pick as her running mate: 

Democrats face 'headwinds' in Midwest

Kamala Harris should pick a centrist running mate who can help her overcome some of the 'significant headwinds' that Democrats face in battleground states, most notably in the Midwest, says Democratic strategist Brett Bruen. The party must win back voters 'who are sitting in the middle, sitting on the fence,' he said.

With the president taking himself out of the running and endorsing Harris as his successor on Sunday, this year's convention can now go one of two ways: either the party unites around Harris as its nominee, or she will have to compete with other potential nominees in an open contest. 

The convention in 1968 wasn't an open contest, but it bears some similarities to this year. The sitting president, Lyndon B. Johnson, also bowed out of the race and endorsed his vice-president as his replacement — though he did so months earlier than Biden.

Sen. Hubert Humphrey easily secured the party nomination when delegates convened in Chicago that August, but he had anything but a united party behind him.

His victory was largely overshadowed by internal fighting over the party's stance on the Vietnam War and violence unfolding on the streets outside the convention hall.

Democrats will be looking to avoid a similar scenario in four weeks' time.

According to Kevin Boyle, an American history professor at Northwestern University, they will instead be looking to nominate their candidate properly under party rules — both to regain a sense of normalcy and to avoid Republican criticism that the Democratic ticket was determined by an undemocratic process.

"They want to look as if they have the situation under control. What you don't want is a nominee — and this is what happened to Hubert Humphrey — getting the nomination while the circumstances are chaotic around them," said Boyle.

"What kind of introduction is that to the American electorate?"