Graham Platner has said that he plans to withdraw from the US Senate race in Maine after facing an allegation of sexual assault, shuttering an insurgent campaign that had withstood months of controversy only to implode and imperil Democrats' attempt to regain power in Washington.

"We believe that for the movement to continue, it can't be me and for that reason, we are suspending campaign operations," he said in a video posted on X on Wednesday.

In the clip, Platner also denied sexual assault allegations that sparked recent calls for him to leave the race.

Maine resident who previously dated Platner, accused him of forcing her to have sex in late 2021 despite her repeated objections.

His campaign rapidly lost support across the Democratic Party, from the Washington establishment to progressive lawmakers and activists who had previously defended him.

Platner's exit will most likely force a reckoning within the party, which has been divided between its moderate and progressive factions, when it is struggling to unify during this year's midterm elections.

Maine is considered a key state for control of the narrowly divided Senate, and Democrats were desperate for a candidate capable of defeating Republican Senator Susan Collins.

Divided path

His exit leaves Democrats facing a compressed and politically fraught replacement process.

Progressives have already begun pushing for a nominee who can inherit Platner's movement and policy agenda, while party leaders are under pressure to choose someone able to reassure moderates, independents and women voters in a state where Collins has repeatedly survived difficult races.

Collins, a five-term Republican and one of her party's few remaining moderates in Congress, is among Democrats' top targets this year.

But the turmoil around Platner has given her campaign a reprieve and exposed Democratic divisions over candidate vetting, populism and electability.

In the video, Platner called on "the Democratic establishment" to step up for the "will and the values" of the people of Maine, a largely rural and rugged state.