The Cook Political Report, a top nonpartisan election handicapper, changed its election rating for Tennessee’s Senate race to a toss-up following former Tennessee Gov. Phil Bredesen (D) launching his bid for the seat.

Bredesen, who served as the last Democratic governor of the state, was a top recruit for Democrats in the race to replace Sen. Bob Corker Robert (Bob) Phillips CorkerHas Congress captured Russia policy? Tennessee primary battle turns nasty for Republicans Cheney clashes with Trump MORE (R-Tenn.), a President Trump Donald John TrumpOmar fires back at Trump over rally remarks: 'This is my country' Pelosi: Trump hurrying to fill SCOTUS seat so he can repeal ObamaCare Trump mocks Biden appearance, mask use ahead of first debate MORE surrogate-turned-critic who won’t seek reelection in 2018.

Bredesen won his first term as governor in 2002 and handily won reelection in 2006 with 69 percent of the vote, winning every county in Tennessee.

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He officially jumped into the Senate race Thursday morning with an online video, arguing that he has the “actual track record that it will take to start working across party lines, to fix the mess in Washington and bring common sense back to our government.”

But Bredesen hasn’t held office since he left the governor’s mansion in 2010 due to term limits. He would turn 75 a few weeks after the 2018 elections.

And any Democrat who wins the nomination would have an uphill climb against a Republican in Tennessee, which hasn’t elected a Democrat to the upper chamber since 1990.

Jennifer Duffy, a Cook Political Report analyst of Senate and gubernatorial races, argues that this ratings change also changes the calculus of the 2018 Senate map. Arizona and Nevada have been seen as the only two realistic Senate seats held by Republicans that Democrats have a shot at flipping.

But moving Tennessee to the toss-up column gives them opportunities in three GOP-held seats, which is the number Democrats would need to take back the Senate. Still, capturing the majority will be a tough feat for Democrats, who will be defending 10 seats in states that Trump carried last year.

“Until now, it has been mathematically impossible as only two GOP seats have been considered truly vulnerable,” Duffy wrote. “This does not mean that Democrats will win the majority; only that it is now mathematically possible.”

“However, if another GOP-held seat or two come into play, it becomes a much more realistic possibility. In the current political environment and given former White House aide Steve Bannon's effort to blow up the party, Democrats appear to be preparing for exactly that scenario.”

Bredesen will face James Mackler, an attorney and Iraq War veteran, in the primary for the Democratic nomination. Chattanooga Mayor Andy Berke has also been considering a campaign.

Republicans will also have a competitive primary, with GOP Reps. Marsha Blackburn Marsha BlackburnTaylor Swift on National Voter Registration Day: 'We need everyone' Democrats smell blood with new DHS whistleblower complaint Hillicon Valley: Election officials prepare for new Russian interference battle | 'Markeyverse' of online fans helps take down a Kennedy | GOP senators unveil bill to update tech liability protections MORE and Stephen Fincher Stephen Lee FincherTrump announces, endorses ambassador to Japan's Tennessee Senate bid Lamar Alexander's exit marks end of an era in evolving Tennessee Tensions on immigration erupt in the House GOP MORE as the top two competitors.