A progressive advocacy group on Tuesday accused former Maine Senate President Troy Jackson of striking a female colleague with a bottle he threw during a state Senate caucus dispute years ago, complicating his emergence as the leading Democratic contender to replace Graham Platner on the November ballot against Republican Sen. Susan Collins.

Progressive Victory said it began contacting sources in Maine after Jackson's name surfaced repeatedly as a possible replacement for Platner, whose Senate campaign is collapsing under a sexual assault allegation from a former girlfriend.

In a post on X, the group said Jackson, "in a heated disagreement, struck a female colleague with a bottle he threw at her" during a caucus meeting when he served as Senate president, and it described the episode as "a widespread open secret" in Maine politics that was "not an isolated incident."

The group conceded that people close to Jackson were denying the account.

An insider on his team and the administrator of a Jackson-affiliated Discord server told Progressive Victory the story was inaccurate, but the group called the denial "not a credible refutation on its own" and said it was reaching out to additional witnesses.

Jackson, a Democrat from Allagash, served as Maine Senate president from 2018 to 2024 and lost the state's June gubernatorial primary to Hannah Pingree.

He told the Bangor Daily News on Monday that he believes he is "the best person to replace" Platner if the nominee steps aside, and on Tuesday he registered the Troy Jackson Senate Exploratory Committee with the Federal Election Commission, giving him the ability to raise money while he weighs a run.

Pressure on Platner mounted the same day when Sen. Bernie Sanders, I-Vt., an early backer, said he had privately urged Platner to exit.

"In light of these very serious allegations, I have recommended that he step aside," Sanders wrote.

Sen. Elizabeth Warren, D-Mass., Rep. Ro Khanna, D-Calif., Sen. Ruben Gallego, D-Ariz., and the Democratic Senatorial Campaign Committee also called for Platner to withdraw, and the DSCC said it would not spend in Maine while he remained on the ballot.

The window is narrow.

Under Maine law, Platner must withdraw by 5 p.m. on Monday, July 13, for the state Democratic Party to substitute a nominee. The party has until July 27 to select one.

Platner, who denies the allegation, said in a video posted on X on Monday that his campaign was "taking the time to reflect on the best path forward."

Maine remains one of the Democrats' top pickup targets in their bid to flip the Senate.