Former presidential hopeful Gov. Steve Bullock‘s decision to run for the U.S. Senate may increase the Democrats’ odds of flipping the chamber blue.

Bullock ended his two-term stint as Montana governor last year, and is widely viewed as the Democrat with the best chances of defeating freshman incumbent Sen. Steve Daines (R-Montana).

Democrats will need to win an additional three or four seats, depending on if their side wins the presidential election or not.

According to the nonpartisan Cook Political Report, Democrats have the best chances of flipping four GOP seats: Arizona Sen. Martha McSally, Colorado Sen. Cory Gardner, Maine Sen. Susan Collins and North Carolina Sen. Thom Tillis.

However, Sen. Doug Jones (D-Alabama) is likely in for a tough fight. Jones beat controversial Roy Moore by 1.7% in a special election after Jeff Sessions resigned to become U.S. Attorney General. Moore, who was accused of sexual misconduct by several women when they were teenagers, did not qualify for the GOP runoff this year, making Jones’ reelection odds significantly less promising.

With Bullock’s Senate bid, which he is expected to announce before Monday’s filing deadline according to CNN, Democrats have a little more leeway for error.

In addition to Montana, two other seats have some chance of being reclaimed by a Democrat: Georgia Sen. Kelly Loeffler‘s seat and the open Kansas seat. Loeffler was appointed to the Senate by Republican Gov. Brian Kemp after former Sen. John Isakson (R-Georgia) resigned at the end of December due to health concerns.

Republican Sen. Pat Roberts‘s (R-Kansas) retirement has prompted a fierce fight for the open seat. A poll released mid-February by SMART, a union representing transportation workers, showed the leading Republican and Democratic candidates deadlocked at 43% support.

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While no seat is guaranteed to flip and the Democrats will have to fight tooth and nail if they want to reclaim the Senate majority, there are two most likely scenarios of the Senate flipping: Jones is reelected and the four GOP seats most likely to flip become blue, or Jones loses and Democrats manage to win five Republican seats from Arizona, Colorado, Maine, Montana, Georgia, North Carolina or Kansas.