Coquitlam River

National Democrats cheered when Mary Peltola, the former Alaska congresswoman, declared in January that she would run for the Senate.

Ms. Beltola, an Alaska native who grew up fishing for salmon on the Coquitlam River, was well-known and well-liked statewide, even by Republicans.

Her party hailed her as the strongest prospect for ousting Republican incumbents and winning control of the Senate.

But behind the scenes, her political operation has been marked by tumult.