In a recruiting victory for Democrats, former Gov. Phil Bredesen announced today that he will run for the open seat created by the retirement of GOP U.S. Sen. Bob Corker. Bredesen's announcement puts the race into the Toss Up column, a rating change that has larger implications for the 2018 Senate map.

Bredesen was Mayor of Nashville from 1991 until the end of his second term in 1999. He ran for Governor in 2002, winning with 51 percent of the vote, and was re-elected in 2006 with 69 percent and winning every county in the state. Bredesen is the last Democrat to win statewide office. Bredesen's electoral road hasn't been without its potholes. He made unsuccessful bids for Nashville Mayor in 1987, a special congressional election in 1987, and Governor in 1994.

Many Democratic strategists believe that Bredesen can appeal to voters across party lines in a state that has become increasingly more Republican in recent years. He also has a demonstrated ability to raise money (and has plenty of his own) and the remnants of a political organization.

Democrat James Mackler, an attorney and Iraq War veteran, has been in the race for months. While Mackler is getting good reviews, he is a political newcomer, who remains largely unknown to voters and has struggled with fundraising. At the end of the third quarter, he had yet to raise $1 million, and had $320,788 in the bank as of September 30. Some of Mackler's supporters take issue with the idea that Bredesen is the party's best bet in the Senate race, arguing that voters are ready for new blood. One even referred to the former Governor as the Evan Bayh of 2018, a reference to the disastrous comeback attempt former U.S. Sen. Evan Bayh made in Indiana in 2016. There are other Democrats who say, though, that they think that Mackler, who is 44 years old, has a bright future and losing a primary to Bredesen is not something he needs on his political resume.

Republicans are slated to host a competitive primary between U.S. Reps. Marsha Blackburn and Stephen Fincher. While other candidates might enter the contest, Blackburn and Fincher are clear frontrunners.

The national significance of Bredesen's candidacy and the race's move to Toss Up is that it puts three Republican-held seats in play. (For the purposes of this math equation, we are excluding Tuesday's contest in Alabama.) As the Senate stands today, Democrats need a net gain of three seats to win the majority. Until now, it has been mathematically impossible as only two GOP seats have been considered truly vulnerable. This does not mean that Democrats will win the majority; only that it is now mathematically possible. At this point, if Democrats want to win the majority, they will need to hold on to their own 25 seats, and then pick up the Republican-held in Arizona, Nevada and Tennessee. That is a tall order. 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.

For Republicans, this seat which seemed like a very safe bet at the start of the cycle is now one of its most vulnerable.