(CNN) Former Montana Gov. Steve Bullock's decision to reverse course and run for the Senate this November is a massive recruiting coup for Democrats that not only makes the party competitive in that state but also improves their odds of retaking the majority this fall.

Bullock, who left office last year as a popular two-term governor, is widely regarded as the only Democrat with a credible chance of beating freshman Republican Sen. Steve Daines in a state that tends to favor Republicans at the federal level. (Bullock, who ran briefly for president in this election, had repeatedly insisted that he had no interest in the Senate. He has apparently changed his mind.)

A Bullock run, which he is expected to announce sometime before Monday's filing deadline, will have reverberations well beyond the Last Best Place.

Before Bullock's decision, non-partisan handicappers -- like the Cook Political Report -- rated three GOP seats as deeply imperiled: Arizona Sen. Martha McSally, Colorado Sen. Cory Gardner and Maine Sen. Susan Collins. But Democrats also have to defend Sen. Doug Jones (D) in GOP-friendly Alabama, a prospect that got much harder when uber-controversial Roy Moore didn't make the GOP runoff earlier this week.

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