Washington — President Biden’s announcement Sunday to end his 2024 reelection bid caps a chaotic few weeks in which he faced calls from more than three dozen elected Democrats to exit the presidential race due to concerns about his fitness for a second term.

Mr. Biden has thrown his support behind Vice President Kamala Harris, and key Democrats are lining up behind her to become the Democratic presidential nominee. Harris said in a statement that she intends to “earn and win” the party’s nomination.

The shakeup at the top of the ticket comes just weeks before Democrats are poised to convene in Chicago for the Democratic National Convention, beginning Aug. 19. The party has long been planning a virtual roll call vote to formally nominate its presidential candidate to take on former President Donald Trump in November, and delegates and party leaders have been told it’s expected to take place Aug. 1.

If that is the case, Democrats would have their presidential nominee before the convention kicks off later that month.

But if that virtual roll call vote doesn’t end up happening at that time, Democrats could be facing an open convention, which happens when there is no set nominee when the convention kicks off. If that happens, candidates vying for the nomination would work to convince delegates — who will select the nominee at the convention — to vote for them.

A candidate needs to win 1,968 delegates, a majority of pledged delegates, in the first round of voting to lock up the Democratic nomination. Democrats have two types of delegates: Pledged, who must support the candidate voters of their state selected in the primaries and caucuses; and superdelegates, party leaders who are not committed to any candidate and cannot vote on the first ballot at the convention.

Mr. Biden was heading into the convention with nearly 3,900 pledged delegates, but they can now vote for the candidate of their choosing. They do not have to support Harris.

Still, Alan Clendenin of Florida, a DNC executive committee member, told CBS News on Sunday that he expects a “supermajority” of delegates to coalesce around Harris in the coming days.

Democratic National Committee chair Jaime Harrison said Sunday that the party will undertake in the coming days “a transparent and orderly process to move forward as a united Democratic Party with a candidate who can defeat Donald Trump in November.

It’s unclear whether Harris will face any opponents in the fight for the Democratic nomination. A number of the party’s rising stars who were seen as possible candidates to step in if Mr. Biden withdrew from the race have endorsed Harris to lead the ticket, including California Gov. Gavin Newsom and Pennsylvania Gov. Josh Shapiro.

Nomination fights at the convention can be long and grueling. At the 1924 Democratic National Convention, it took a record 103 ballots to nominate John W. Davis for president and Charles W. Bryan for vice president. The pair went on to lose that November.

Aaron Navarro contributed to this report