The president's tweet of support for the congresswoman came a day after outgoing GOP Sen. Bob Corker said he would not campaign against former Gov. Phil Bredesen, the leading Democratic candidate in the race. Trump called Blackburn a "wonderful woman who has always been there when we needed her."
Trump aims to give a boost to Blackburn in a red-state race Republicans cannot afford to lose in November. She became the preferred Republican candidate in the race after Corker declined to run for a third term. Trump had pushed Corker, an ally of the president who occasionally offers criticism of him, to run for office again.
Republicans hope to keep or expand their 51-seat majority in the Senate and have a favorable path to doing so as 26 Democrats or independents who caucus with them face re-election. Only nine Republican incumbents have to run this year.
It is unclear whether Trump's presence in the race would help or hurt Blackburn during an election year in which many voters have pushed back against Trump and GOP policies. Fifty percent of Tennessee adults said they approve of Trump, while 44 percent said they disapprove, according to Gallup survey results published in January.
Blackburn, who has run as a pro-Trump candidate, thanked Trump in a statement for "both his support and leadership."
"Tennesseans from all walks of life support the President's work to cut taxes, rebuild the military, care for our veterans, and nominate conservative judges who won't legislate from the bench," she said in a statement. "They expect Tennessee's next Senator to work alongside the President and their Senate colleagues to pass his agenda, and I'm looking forward to doing exactly that."
Bredesen, who served two terms as Tennessee's governor, faces a challenge to win in a state Trump carried with more than 60 percent of the vote in 2016. However, comments from Corker this week and a recent poll suggest the race may be more competitive than Republicans would hope.
On Wednesday, Corker told reporters he thinks Bredesen has a "real" 6-percentage-point advantage in the race. Polling has been scarce so far, but one Middle Tennessee State University survey in late March showed a 10-point edge for Bredesen.
Corker called Bredesen a "friend" with whom he has worked for more than 20 years. The senator said Wednesday he does not plan to attack the former governor.
"I'm not going to campaign against someone who I've been a friend with and worked with, you know? So that's the way it's going to be," Corker said at a Christian Science Monitor breakfast.
Corker has shown some support for Blackburn, who has served in Congress since 2003. On Wednesday, he said he gave her campaign an undisclosed contribution. He added that he is "supportive."
However, he said he had a stronger working relationship with Bredesen than he did with Blackburn.