Graham Platner suspended his campaign for Senate in Maine on Wednesday, bowing to intense pressure from party leaders after a rape allegation he denies, and opening an uncertain and highly unusual chapter for Maine Democrats.

Mr. Platner’s exit leaves the Maine Democratic Party less than three weeks to find a replacement to challenge Senator Susan Collins, a vulnerable Republican, in one of the country’s most competitive battlegrounds.

Here’s what comes next:

When and how will Democrats pick a new nominee?

There is no set process by which the state party must choose a replacement, but its leaders said in a statement late Wednesday that it would hold some form of nominating convention.

The deadline to find a new nominee is July 27 under state law. The state party leaders said in their statement that they would soon announce a “full timeline, details for how the nomination process will move forward, information about how to participate, and requirements for candidates.”

Who is angling for the nomination?

Even before Mr. Platner bowed out, a crowd of potential contenders to replace him had already begun to form. And some candidates have already officially said they will run.

Troy Jackson, a former president of the Maine Senate favored by some progressives, announced his bid Wednesday night, as did Jordan Wood, a progressive who lost last month in the House primary in northern Maine’s swing congressional district. Dan Kleban, a founder of a brewery, has also said he will run, writing in a Substack post, “I’m in.”

Other prospective candidates include Nirav Shah, a former state and federal public health official; and Shenna Bellows, the Maine secretary of state.

Mr. Shah, Mr. Jackson and Ms. Bellows all lost in the state’s ranked-choice Democratic primary for governor last month.

Mr. Jackson said in a statement that there “is a powerful movement of working class people in the state of Maine, and millions more across America who are ready to send a progressive fighter to the Senate.”

“I’ve been fighting for that movement my whole life — and I’m sure as hell not backing down now, when this fight is needed most,” he added.

Mr. Jackson had already won the support of Representative Ro Khanna, a progressive California Democrat who was once one of Mr. Platner’s most vocal endorsers. “I am all in for Troy Jackson to continue the populist progressive movement in Maine,” Mr. Khanna said in a statement Wednesday afternoon.

Mr. Wood, who was once chief of staff to Representative Katie Porter of California, also quickly moved to appeal to progressives.

“I’m running for U.S. Senate because to beat Susan Collins, Democrats need a candidate who can provide a true contrast and run an unapologetically progressive campaign,” Mr. Wood, who cut a trip to North and South Dakota short this week, said in a text message from the air as he flew back to Maine. “Passing Medicare for All. Stopping ICE terrorizing our streets. Standing up to Donald Trump’s abuse of power. I am that candidate.”

Does Platner have any influence on who takes his place?

The state party, in its statement, credited Mr. Platner, an oysterman running a progressive, outsider campaign, for helping to drive an “unprecedented amount of energy and enthusiasm among Maine Democrats.”

But the state party, which also called on Mr. Platner to leave the race by Monday, has also been at odds with him over the process to select a replacement.

The party’s executive director, Devon Murphy-Anderson, said Tuesday that Mr. Platner’s team had been trying to “put their thumb on the scale” in the party’s planning for his replacement. She said he would have no role in the process.

It was not clear if Mr. Platner would attempt to influence the process going forward. In a video announcing his exit, Mr. Platner, who won the nomination last month, said the process should be driven by the voters who turned out in the primary and “voted against the political system, against the donor class.”

“I’m not trying to dictate to anyone who it should be, or how we get there,” Mr. Platner said. “But I will say this: It needs to be open, transparent and democratic. It needs to be reflecting the will and the values of the people that built this movement.”

The state party chair, Charles F. Dingman, a progressive, said Wednesday that the party had not received any further outreach from Mr. Platner’s team.

The state party’s statement called for unity, saying that it looked forward to “coming together and harnessing” energy created around Mr. Platner’s campaign.

Its membership held a meeting Wednesday evening where the form of convention was discussed. But Mr. Dingman emphasized hours later that the exact contours of the process had not been finalized.

“We are going to have a nominating convention,” he said by phone, “and it is going to be representative.”