Schweitzer is Democrats' best hope at winning retiring Sen. Max BaucusMax BaucusGOP hasn’t reached out to centrist Dem senators Five reasons why Tillerson is likely to get through Business groups express support for Branstad nomination MORE's (D-Mont.) seat. He told The Hill shortly after Baucus's retirement that he was looking at a run, but has been silent since — which has made some Democrats nervous.

But Tester isn't one of them, despite not having talked to Schweitzer in some time.

"I will tell you, I haven't talked to Brian in six months, but the truth is he's working on a mine issue right now. I anticipate, and my crystal ball is still a little cloudy, but I anticipate he's going to get in to this thing."

If the popular former governor runs he'll likely have the edge in a race — though former Gov. Marc Racicot (R), who hasn't ruled out a bid, polls strongly against Schweitzer. 

But if he sits it out, Democrats will face an uphill battle to hold the seat, giving the GOP an easier path to winning back control of the Senate.

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