Long before Donald J. Trump upended the Republican Party, Scott Reed was one of its go-to political operatives. One of his closest relationships was with Senator Susan Collins of Maine, and her husband, Tom Daffron. Mr. Daffron previously worked at Mr. Reed’s government relations firm, and last year it paid him for consulting. Mr. Reed has also raised millions of dollars to help Ms. Collins beat back challengers.

The longstanding ties are now under intense scrutiny following a report by Pro Publica that said the F.B.I. had sought to begin a pay-to-play investigation examining Ms. Collins’s interactions with a donor seeking federal contracts. Mr. Reed exchanged emails with the donor, Martin Kao, according to Pro Publica. The New York Times confirmed that the F.B.I. had begun investigating Mr. Kao’s allegations that he and Mr. Reed had discussed circumventing campaign finance laws.

Media coverage arrives at a fraught moment for Ms. Collins, who is in one of the tightest Senate races. She called the article “truly outrageous” and attacked the credibility of Mr. Kao, who is serving an 87-month sentence. Mr. Reed said he never communicated with Ms. Collins about Mr. Kao. Ms. Collins’s Democratic rival, Troy Jackson, is seizing on the allegations.