President Joe Biden’s announcement Sunday he won’t seek the nomination that he won this year in primaries and caucuses in every state, including where his name wasn’t on the ballot, puts the Democratic Party in uncharted territory just four weeks before delegates are due to hold their national convention in Chicago.

Biden threw his support behind Vice President Kamala Harris to take on Republican nominee Donald Trump, but that tweet alone does not settle anything.

Here’s a look at some of the questions that lie ahead, and things to watch along the way.

Who runs?

Harris on Sunday afternoon quickly ended any speculation about her plans. “I am honored to have the President’s endorsement and my intention is to earn and win this nomination,” she said in a statement released by the Biden-Harris campaign about two hours after Biden’s bombshell announcement.

“Over the past year, I have traveled across the country, talking with Americans about the clear choice in this momentous election,” the former senator and California state attorney general added. “And that is what I will continue to do in the days and weeks ahead. I will do everything in my power to unite the Democratic Party — and unite our nation — to defeat Donald Trump and his extreme Project 2025 agenda.”

Biden had flatly rejected calls to step aside for weeks after a disastrous debate with Trump on June 27.  White House and campaign aides had not provided requested details on what changed his mind. A White House official said Sunday that Biden kept Harris in the loop, saying the duo “spoke multiple times earlier today ahead of his announcement.”

On a Saturday morning call with reporters, Biden campaign aides said only that Biden, who has been convalescing at his Delaware beach house after contracting COVID-19 while on the campaign trail, was feeling better. They added his intention was to fully recover and get back to campaigning.

No other Democratic officials immediately announced plans to run. 

“My job in this election will remain the same: doing everything I can to elect Democrats and stop Donald Trump,” Michigan Gov. Gretchen Whitmer, a national co-chair for Biden’s campaign, said in a statement. Whitmer previously said she wouldn’t run, even if Biden dropped out of the race. 

Illinois Gov. J.B. Pritzker, who has been mentioned as a potential candidate, issued a statement praising Biden and attacking Trump but did not mention Harris.

California Gov. Gavin Newsom, after initially issuing a statement that didn’t mention her, later said on X that “no one is better to prosecute the case against Donald Trump’s dark vision and guide our country in a healthier direction than America’s Vice President, @KamalaHarris.”

How will it work?

The party’s elected delegates are now poised to have perhaps a more interesting task in Chicago next month than they may have expected. 

Elaine Kamarck, a senior fellow in governance studies and the director of the Center for Effective Public Management at Brookings, said on Friday a Biden withdrawal could trigger a condensed version of a primary if multiple candidates came forward. But it would be the convention’s delegates who would be the courted voters.

“What would happen is we would see who wants to run. We don’t really know that yet,” she said. “We have all sorts of people [who] are being mentioned to run, but none of them have said they want to run. In fact, they’ve said quite the opposite.”

Kamarck made those comments during a webinar Friday hosted by the new group Delegates are Democracy, which was created to educate delegates on their role in the party’s nominating process. 

Candidates seeking the nomination first need to gather signatures from at least 300 delegates, with a maximum of 50 from each state. 

“I think you’d have some kind of campaign going on. And it would be public because in fact the delegates are listening to the public, but it would be really focused on the delegates themselves,” Kamarck said. “Then when you got to the convention, the question would be do we have a consensus? Is there a first ballot nomination or not, or would you maybe need a couple ballots to get a nominee?”

Democratic National Committee Chair Jamie Harrison said in a statement that the party would “undertake a transparent and orderly process to move forward as a united Democratic Party with a candidate who can defeat Donald Trump in November.”

“This process will be governed by established rules and procedures of the Party. Our delegates are prepared to take seriously their responsibility in swiftly delivering a candidate to the American people,” he said. 

The Democratic National Convention Rules Committee is set to meet in the coming days after a Friday meeting in which they took no votes. The panel has been weighing whether to move forward with a virtual roll call vote in early August before the convention. Some advocates for a virtual roll call vote are concerned with potential legal issues if the candidate isn’t officially nominated before an earlier date than the convention. Others are pushing for an open process at the convention. 

What about Biden’s campaign account?

Biden and Harris were both listed as candidates on the organization statement for Biden for President, an account that had $96 million on June 30, according to the latest disclosure to the Federal Election Commission.

“The Vice President and her running mate can continue using the campaign’s existing funds for the general election if she is on the Democratic ticket as either the presidential or vice-presidential nominee,” Trevor Potter, a former FEC chairman, said in a statement from the Campaign Legal Center, where he is president.

If Harris is not on the ticket, the Biden campaign could refund money to donors, or give it to the national party or state parties that could spend some of it to coordinate with the presidential ticket, Potter said.

Who’s with Harris and who isn’t?

Former President Bill Clinton and former Secretary of State Hillary Clinton said in a tweet they were “honored” to join Biden in endorsing Harris “and will do whatever we can to support her.” Former President Barack Obama, however, said in a statement that he has “never admired Joe Biden more than I do today” — but did not mention Harris.

Some top congressional leaders stopped short of endorsing Harris. In fact, in separate statements, Senate Majority Leader Charles E. Schumer of New York, House Minority Leader Hakeem Jeffries of New York and Nancy Pelosi of California all praised Biden — but did not mention the vice president.

One prominent pro-Biden super PAC quickly signaled its support for Harris. Unite the Country, which raised and spent nearly $50 million in support of Biden, said Sunday it “stands behind — and will fight everyday — to elect @KamalaHarris as the next President of the United States.”

Sen. Chris Coons, D-Del., a close Biden ally, said he spoke to the president on Sunday, but declined to say why Biden reversed course. He did say that Biden expressed “confidence” in Harris, with Coons echoing that sentiment.

“I support Vice President Harris. I am very hopeful that we will come out of our convention next month united,” Coons told CNN. “And we’ll see what happens in terms of the choice for a running mate. But I think our president, Joe Biden, has made his endorsement clear. And I think that is the most important endorsement that we could all be paying attention to today.”

Some other Biden backers, including Rep. James Clyburn of South Carolina, also quickly backed Harris. And some made sure Democrats saw the potential consequences of going another route.

“When we say trust Black women, we mean it,” said Rep. Cori Bush, D-Mo., a progressive facing a primary challenge. “Black women are the backbone of the Democratic Party and it is past time for us to lead our country forward. Kamala Harris is more than ready to lead this moment.”

The Congressional Black Caucus was also quick to endorse Harris, saying she “has been instrumental in delivering the accomplishments of the last 3.5 years” and “will do an excellent job as president of the United States.”

Harris, who served in the Senate from 2017 to 2021, was endorsed by a slew of senators who hold safe seats or are not up for reelection this cycle, including Christopher S. Murphy of Connecticut, Tina Smith of Minnesota, Elizabeth Warren and Ed Markey of Massachusetts and Patty Murray of Washington state.

Sen. Tim Kaine of Virginia and Martin Heinrich of New Mexico, who are both up this cycle, endorsed Harris, as did Tammy Baldwin of Wisconsin, who is facing a tough reelection battle.

Other members also held off in their initial statements. Sen. Peter Welch of Vermont, the first Democratic senator to publicly call for Biden to quit the race, said the party has “many capable, proven leaders — Vice President Harris among them. We have a deep bench of leaders who will carry on the Biden legacy.”

Sen. Bernie Sanders, the Vermont independent who battled both Biden and Harris for the nomination in 2020, had stood by Biden while others called for him to abandon his campaign. He publicly thanked the president on X for serving “with honor and dignity,” but made no mention of Harris.

Who runs with Harris?

She has not said, but names that have been mentioned include Sen. Mark Kelly of Arizona and Govs. Andy Beshear of Kentucky, Roy Cooper of North Carolina and Josh Shapiro of Pennsylvania. 

Harris and Cooper were both state attorneys general at the same time and were together at a campaign event on Thursday.  

“I’ve known him for almost two decades,” Harris said in Fayetteville, N.C. “And he is an extraordinary leader.”

Kelly said on X he “couldn’t be more confident that Vice President @KamalaHarris is the right person to defeat Donald Trump.” His wife, former Rep. Gabby Giffords, who now leads a national gun control organization after being shot in an assassination attempt, put out her own statement saying Harris “will make a great president.”

How will it play?

Republicans argued Biden’s decision was a sign he was unable to run, and argued he therefore should not continue to serve and Democrats who stood by him were colluding to deceive the public. 

They also quickly turned their attention to Harris — including Trump, who cast aside talk that he would run as a more unifying figure following the assassination attempt against him last weekend.

“And during this entire term, Kamala Harris — as well as every other Democrat in Washington, sat by and did NOTHING. They are all just as complicit as Biden is in the destruction of our once-great Nation, and they must all be thrown out of office,” Trump said in a Sunday statement.

“Kamala Harris is just as much of joke as Biden is. Harris will be even WORSE for the people of our Nation than Joe Biden. Harris has been the Enabler in Chief for Crooked Joe this entire time,” he added. “They own each other’s records.”

GOP strategist Ford O’Connell said in a Sunday email that the push to oust Biden showed “the party that champions democracy appears indifferent to the will of the voters.” 

Biden’s strongest backers also are unlikely to forget how the president was treated by Democrats who made public statements and anonymous quotes to news outlets.

“Now that the donors and electeds have pushed out the only candidate who has ever beaten Trump, it’s time to end the political fantasy games and unite behind the only veteran of a national campaign — our outstanding @vp, @KamalaHarris!!” Ron Klain, who served as Biden’s first chief of staff in the White House, wrote on X. “Let’s get real and win in November!”

What to watch

Biden promised a national address that could lay out his reasons in greater detail than the letter he issued Sunday. That may or may not ease arguments he is not fit to continue serving.

Harris will also get new scrutiny from this day forward. Her first major decision, choosing a running mate, has the potential to be a boost for the campaign or foreshadow more second-guessing.

The whole effort to switch candidates is also likely to face legal challenges.