PORTLAND, Maine - Democrats across Maine started positioning themselves Thursday to become the replacement nominee for a critical U.S. Senate race following the withdrawal announcement by progressive candidate Graham Platner amid a sexual assault allegation.

State law requires Democrats to select a new candidate by July 27. The chosen nominee will have under four months to challenge longtime Republican Sen. Susan Collins in November. Several potential contenders had already signaled interest before Platner, who rejects the claim, confirmed plans to step aside. He is set to submit formal withdrawal documents on Monday, the final day allowed.

A number of candidates moved quickly to declare their bids on Thursday.

The Maine Democratic Party plans to hold a convention with hundreds of delegates from around the state to pick the nominee, though details on timing and format are still unclear.

Maine remains a crucial battleground for Senate control, and Democrats seek a strong contender to take on Collins at a time when President Donald Trump faces widespread disapproval.

Gov. Janet Mills, who suspended her primary bid in late April, has not signaled whether she will reenter the contest.

Here are the main figures who have expressed interest in the Senate race:

Jackson, the former state Senate president, lost his bid for the Democratic gubernatorial nomination earlier this year despite support from Platner and U.S. Sen. Bernie Sanders. He launched his Senate campaign shortly after Platner stepped aside, emphasizing his role as a progressive advocate. The group Our Revolution, started by Sanders, announced its endorsement of the 58-year-old.

Jackson issued a statement Thursday that included endorsements from numerous current and former state and local leaders.

Shah, who previously led Maine Center for Disease Control and Prevention, declared his candidacy Thursday. He finished second in the Democratic primary for governor and positioned himself as a moderate alternative to Jackson. During a news conference, Shah invited Platner backers to support his effort.

"You have an important place in this campaign and we welcome your voices," Shah said. "This campaign represents the values that we all care about."

Kleban, co-founder of Maine Beer Company, confirmed his run on Wednesday. He had briefly entered the Senate contest last year but withdrew when Mills launched her bid. The 49-year-old backed Mills before she exited the primary.

"I'm ready to fight for Mainers and bring a new generation of leadership to Washington," Kleban said.

Bellows, Maine secretary of state, announced her candidacy Thursday and highlighted her record of handling difficult challenges, including her work as a civil liberties advocate and her disputes with President-elect Donald Trump on ballot issues.

This marks her second attempt at statewide office. Bellows, 51, finished fourth in the Democratic primary for governor in June. She also ran against Collins in 2014 as the Democratic Senate nominee and suffered a decisive defeat.

Wood, 36, tried to enter the Senate primary last year but switched to the 2nd District race, where he placed third. He has now expressed renewed interest in the Senate and formally announced on Thursday.

"To beat Susan Collins, we need a candidate who can provide a true contrast and run an unapologetically progressive campaign: Passing Medicare for All. Stopping ICE terrorizing our streets," Wood posted on social media Tuesday.

Loud submitted filing paperwork earlier this week. The 29-year-old social worker also competed in the 2nd District Democratic primary but finished last in the initial round of ranked-choice voting.

Geiger, a state lawmaker and former Platner supporter, is considering a run. In a Wednesday interview, the 70-year-old said she would retain Platner's campaign staff, calling them "deeply impassioned and confident young people."

Costello, who placed third in the June primary behind Platner and Mills, announced his return to the race on Thursday. In a social media post, he argued his background aligns with the daily struggles of many Mainers.

Kruesi reported from Providence, R.I.