Bloomberg's involvement is likely to increase scrutiny on one of the tightest races in the midterm elections, as Bredesen, a former Tennessee governor, aims to flip a Republican-held seat representing a state Donald Trump won easily in 2016.
The event is being billed to financiers as an opportunity to "join Mike Bloomberg for an evening in support of Governor Phil Bredesen" on Oct. 9, according to the invitation. A person who plans to attend the event said Bloomberg will host the fundraiser at his home in Manhattan.
Donors are being asked to contribute up to $2,700 to Bredesen's Senate campaign in order to attend.
Representatives for Bredesen and Bloomberg did not return requests for comment.
The campaign for Republican candidate Rep. Marsha Blackburn seized on the news to paint Bredesen, who served two terms as Tennessee governor, as an outsider beholden to national Democratic interests.
"Phil Bredesen is bought and paid for by Chuck Schumer and Mike Bloomberg. Now, he is returning to his home state to fundraise with another anti-gun, pro-tax and spend it all billionaire," Abbi Sigler said in an email. "The similarities are striking, and this fundraiser is another clear indication that Phil Bredesen will not stand up for Tennesseans' Second Amendment rights."
Bredesen grew up just 30 miles outside of Rochester, NY. It's unclear whether he will be attending Tuesday's event.
The news comes just a day after Bloomberg announced he would be contributing $20 million to the Senate Majority PAC, a group dedicated to electing Democrats to the U.S. Senate and that is affiliated with allies of Minority Leader Chuck Schumer.
Republicans hold a 51-49 majority in the Senate. Pundits and election analysts favor the GOP to hold on to the chamber, as 10 Democratic incumbents are running in states Trump won in 2016. Yet Republican-held Senate seats in Tennessee, Nevada and Arizona are also considered up for grabs.
Bredesen is looking for a boost across the finish line in his heated matchup with Blackburn, a veteran Republican House lawmaker. The seat is currently held by Republican Bob Corker, an occasional Trump critic who is retiring.
The two are almost even in the polls and in the fundraising game. The Real Clear Politics polling average has the two candidates virtually tied. The Center for Responsive Politics shows Bredesen out-raising Blackburn by just over $1 million.
Blackburn, however, has the support of Trump, which could be her biggest advantage, as he won the state by 26 percentage points two years ago. The president visited Tennessee on Monday for a rally in support of Blackburn.
"She's got Tennessee values," Trump said at the time. "A vote for Marsha is really a vote for me and everything that we stand for. It's a vote for 'Make America Great Again.'"
Blackburn also has outside money and influence groups, such as the Koch network, rallying to her cause.
Bloomberg's support of Bredesen comes as he amplifies efforts to swing Congress to Democratic control.
Bloomberg, a Republican turned independent, pledged $80 million in June to help Democrats flip the House. Since then, he has headlined numerous fundraisers across the country. Soon, he will head to Pennsylvania to speak at the Montgomery County Democratic Committee's annual fall dinner.
It remains unclear which House candidates will get a boost from Bloomberg's money.