Montana state Sen. Champ Edmunds (R) will officially declare his bid against Sen. Max Baucus (D-Mont.) on Thursday.
Edmunds has been publicly discussing a run against Baucus for some time. He'll be the second Republican to formally enter the race. Former gubernatorial candidate and state Sen. Corey Stapleton (R) already announced his own campaign.
Polls show Baucus is vulnerable to a challenge in both the primary and general election, but he starts off with a big lead against both Edmunds and Stapleton, neither of whom are well-known statewide.
According to a recent survey by Democratic firm Public Policy Polling, Baucus leads Stapleton 45 percent to 38 percent and leads Edmunds 47 percent to 37 percent.