Although Kamala Harris is now the presumptive nominee for the Democratic Party, her road to the nomination at the Democratic National Convention could face legal and political challenges.

Joe Biden and every state Democratic Party director have thrown their support behind Harris, and she has inherited Biden’s election war chest.

Potential political rivals have also lined up to show unity, with the US Vice President winning the endorsement of influential state governors Josh Shapiro, Pete Buttigieg and Gavin Newsom.

Harris will have to navigate a DNC open convention from August 19-22, an event that hasn’t occurred since Hubert Humphrey won the nomination and lost the election to Richard Nixon in 1968.

Humphrey’s nomination was seen as undemocratic and was unpopular with the party members. It led to reforms that allowed state voters to choose who their delegates would vote for.

At this year’s DNC, 4000 delegates, most of whom were previously pledged to Biden, will vote for their preferred candidate.

If there is no majority consensus in the first round of voting, then about 700 “superdelegates”, chosen from a pool of high-profile Democrats, will decide the nomination.

If Harris wins the majority of delegates – as she likely will – when the vote is held, she will assume the mantle as nominee and face down Republican candidate Donald Trump in November.

Several people outside the Democratic leadership have signalled a potential outside bid at the nomination, while the Republican Party’s speaker raised the prospect of challenging her ability to appear on the ballot.

Outside challenge?

Joe Manchin, the former Democratic senator for deeply conservative West Virginia, is mulling a return to his party by challenging Harris for the nomination, according to CNN.

“I’m hoping for that, because I think it will leave him with a tremendous legacy as one of the greatest leaders we’ve had,” Manchin said just hours before Biden left the race.

“I say this, and I came to the decision with a heavy heart, that I think it’s time to pass the torch to a new generation.”

The 76-year-old Manchin quit the Democratic Party earlier in the year to become an independent, while also announcing he wouldn’t seek re-election to the senate, in a blow to the Democrats’ chance of winning the senate.

Joe Manchin is considering challenging Harris for the nomination. Photo: Getty

Robert F Kennedy Jr also expressed interest in contesting the nomination, despite failing to win it from Biden last year, admitting to sexually assaulting a babysitter and having part of his brain eaten by a worm.

Across party lines

Mike Johnson, the Republican Speaker of the House, has claimed that replacing Biden on the ballot in many states would be “unlawful” while raising the prospect of legal challenges.

“It would be wrong, and I think unlawful, in accordance to some of these states’ rules for a handful of people to go in a back room and switch it out because … they don’t like the candidate any longer,” he said on ABC This Week.

“I think there’ll be a compelling case to be made that that shouldn’t happen, and so I think they’ve got legal trouble.”

Election experts, however, poured hot water on Johnson’s nakedly partisan push.

“Joe Biden is not the party’s nominee now,” Richard Hasen, an election expert, told the American ABC News.

“States generally point to the major party’s nominee as the one whose name is on the ballot.”

Biden’s withdrawal has again reshaped a White House contest shaken repeatedly during the past month.

On June 27, Biden’s poor performance in a debate with Trump led many Democrats to urge him to drop out.

Then on July 13, a gunman attempted to assassinate former president Trump.

And last week Trump named hardline Republican US Senator JD Vance, 39, to serve as his vice-presidential running mate.

Harris has been outspoken on abortion rights, an issue that resonates with younger voters and progressives.

She is expected to stick largely to Biden’s foreign policy playbook on such issues as China, Iran and Ukraine, but could strike a tougher tone with Israel over the Gaza war if she tops the Democratic ticket and wins the November election.

-with AAP