President Joe Biden said on Sunday he would withdraw from the 2024 presidential election race, putting the United States into uncharted territory.

Biden endorsed Vice President Kamala Harris to be the Democratic nominee.

Before Biden's decision was made, Reuters spoke to Elaine Kamarck, a senior fellow at the Brookings Institution think-tank, a Democratic National Committee member and author of the book "Primary Politics" about the presidential nominating process, who explained how the process could work. Reuters also spoke to legal experts and Democratic Party officials.

Q: WHAT HAPPENS NOW?

A: Biden has spent the last several months accruing nearly 4,000 Democratic delegates by winning primary elections in US states and territories.

Those delegates would normally vote for him to be the party's official presidential nominee at the Democratic National Convention, which is to take place Aug. 19-22, but the rules do not bind or force them to do so. Delegates can vote with their conscience, which means they could throw their vote to someone else.

By stepping aside, Biden is effectively "releasing" his delegates, potentially sparking a competition among other Democratic candidates to become the nominee.

Within hours of Biden's announcement, Harris' allies were working the phones - calling delegates and party chairs to get their backing, sources told Reuters.

Q: WHO COULD REPLACE BIDEN?

A: Several candidates could step into the fray.

Harris is at the top of the list, but she has had her own problems after a rocky start as vice president and poor polling numbers. The US Constitution dictates that the vice president becomes president if the president dies or becomes incapacitated, but it does not weigh in on an inter-party process for choosing a nominee.

California Governor Gavin Newsom, Michigan Governor Gretchen Whitmer, Kentucky Governor Andy Beshear and Illinois Governor J.B. Pritzker have all been floated as possible replacements. Up until now they have been Biden supporters working to help get him elected, and Whitmer has said she supports Harris.

Q: HOW WILL A NOMINEE BE CHOSEN?

A: There could be a free-for-all of sorts between the Democratic heavyweights vying for the job.

According to Ballotpedia, there are expected to be some 4,672 delegates in 2024, including 3,933 pledged delegates and 739 so-called superdelegates - senior party members.

In order to secure the nomination, a candidate would need to get a majority - that is, more votes than all the others combined.

That's what Harris' allies are trying to do right now - secure the pledged support of 1,969 delegates, and shut down any competition.

If no one achieves that, then there would be a "brokered convention" where the delegates act as free agents and negotiate with the party leadership. Rules would be established and there would be roll-call votes for names placed into nomination.

It could take several rounds of voting for someone to get a majority and become the nominee. The last brokered convention when Democrats failed to nominate a candidate on the first ballot was in 1952.

WHAT HAPPENS TO BIDEN'S CAMPAIGN CASH?

The Biden-Harris campaign had $91 million in the bank at the end of May, but experts on campaign finance law disagree on how readily the money could change hands.

Because Harris is also on the campaign filing documents, many experts believe the money could be transferred over to her if she is on the ticket. There is some debate about whether Biden would need to be officially nominated first as the party's candidate before a transfer could be made.