Graham Platner, the Democratic candidate for U.S. Senate in Maine, is under mounting pressure to exit the race after Politico published a sexual assault allegation against him on Monday.

The woman who made the allegation is identified as Platner's girlfriend, NPR reported. Platner called the claim "Categorically untrue" and, in a separate statement, "Categorically false." He said he was assessing next steps in the wake of the allegation.

Prominent Democrats at both the national and state level called on Platner to withdraw. On NPR, Democratic strategist Joel Payne told host Steve Inskeep that transparency was Platner's only path forward.

The Maine state Democratic Party said Platner's campaign tried to shape the process by which a replacement candidate would be chosen if he left the race. Platner's team denied that claim.

Former Maine state Sen. Troy Jackson has already filed paperwork to take Platner's place on the ballot. Jackson said it would be self-serving for Platner to remain in the race.