Billionaire and former New York Mayor Mike Bloomberg will host a fundraiser for Tennessee Democratic Senate candidate Phil Bredesen, CNBC has learned.

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.