Bredesen, the state's former governor, received about 37 percent of Thursday's vote in defeating Democrats Gary Davis and John McWolfe for the party nomination.
On the Republican side, Blackburn, a member of the U.S. House since 2003, easily defeated primary challenger Aaron Pettigrew with 85 percent support. Pettigrew, a truck driver and transplant from Wyoming who ran as a relative unknown, received nearly 100,000 votes.
The race in November will pit Breseden, a moderate Democrat with a track record of obtaining cross-party votes, against Blackburn, who's considered a staunch conservative eager to receive the support of President Donald Trump.
Tennessee is typically considered a red state, but Breseden's appeal and Trump's relatively low approval ratings are making the contest tight and competitive.
Blackburn, if she wins, would become Tennessee's first female senator. However, experts say this is one race Democrats have a chance at in their fight to win back control of the upper chamber.
"I think it's the measure of dissatisfaction with Blackburn, and Breseden tapped into that pretty effectively," Vanderbilt University political science professor John Geer told the Tennessean. "He's probably ahead by 5, 6, 7 points and, you know, it's going to be a battle."
Tennessee's gubernatorial race also held a primary Thursday.
In the GOP primary, Bill Lee defeated Randy Boyd and Diane Black. Lee and Boyd, both businessmen who self-financed their campaigns, overcame the wealthy, pro-Trump Black despite her status as a front-runner and support from Vice President Mike Pence. Trump stayed out of this race and declined to endorse a candidate.
Boyd spent $19 million of his own money, Lee $5 million and Black $12 million. Nearly $50 million among them was spent to influence voters, the Tennessean reported.
Lee will face former Nashville Mayor Karl Dean in November. He easily defeated Tennessee House Minority Leader Craig Fitzhugh in the Democratic primary Thursday, 75 percent of the vote.