Graham Platner’s insurgent bid for Senate in Maine was gathering steam last summer when his campaign was confronted with some potentially explosive information.

Mr. Platner’s wife, Amy Gertner, told a senior campaign aide that he had been exchanging sexual messages with multiple other women.

It was the kind of revelation with the potential to damage the political newcomer just as his campaign to unseat Senator Susan Collins, a Republican, was beginning to resonate with voters, especially in a state where female voters make up a large share of the electorate.

Mr. Platner’s exchanges with women were confirmed by current and former campaign officials, who gave different accounts of some of the details. Ms. Gertner said the couple, who had married in November 2023, according to the town clerk of Sullivan, Maine, was working through his indiscretions in marriage counseling.

Genevieve McDonald, a former state legislator who was the Platner campaign’s political director before leaving in October, said Ms. Gertner reached out just days before a big Labor Day rally with Senator Bernie Sanders, independent of Vermont, and was concerned her husband’s behavior could become a political liability.

Ms. McDonald said Ms. Gertner told her that her husband had been exchanging sexual messages with as many as a dozen women.