Sen. Bob Corker (R-TN) on Sunday defended himself against criticism over his praise of the Democratic candidate seeking to replace him in the U.S. Senate, while at the same time offering an unenthusiastic endorsement of the Republican nominee.

The senator, who is not running for re-election, accused GOP leaders of leaking the details of a conversation during which Senate Majority Leader Mitch McConnell (R-KY) reportedly rebuked him for speaking positively of former Tennessee Gov. Phil Bredesen, the Democratic nominee for Corker's soon-to-be-vacated seat, at a breakfast with reporters last week.

“It’s hard to believe that the leadership of the senatorial committee on the Republican side would even leak that story out to The Washington Post and cause you to ask me about it,” Corker said on CNN’s State of the Union, referring to the Post’s Thursday report detailing the discussion between McConnell and Corker.

McConnell was angry with Corker for calling Bredesen “a very good mayor, a very good governor, a very good business person.” The majority leader reportedly told Corker that those remarks were unhelpful for the GOP’s chances of retaining control of the chamber.

Corker has said that he wouldn’t campaign against Bredesen because he is a friend. But he has thus far declined to publicly praise Rep. Marsha Blackburn (R-TN), the outspoken Trump booster who is running against Bredesen, in a similar way. While Corker has said he would vote for Blackburn and has donated the maximum amount to her campaign, he offered an unenthusiastic endorsement of the congresswoman.

During appearances on both CNN and ABC, Corker declined to even mention her name, referring to her as “the nominee” and “this person.” When asked why Blackburn would be a better replacement, Corker did not praise her candidacy and instead argued that her election to fill his seat would help Republicans maintain their majority in the Senate.

“I think most people in our state—it is a red state—will focus on the first vote she makes, and that’s the vote to elect the majority leader. And I think at the end of the day that’s going to be a big factor in the race,” Corker, the chairman of the powerful Senate Foreign Relations Committee, said on CNN, later adding: “You know I’m supporting the nominee. I’ve worked with the nominee for some time. And I don’t know what else to say.”

Corker’s refusal to outwardly campaign for the GOP nominee comes at a particularly thorny time for Senate Republicans, who are already fretting over the possibility that Democrats could re-take control of the chamber in a year when the GOP has a built-in advantage.

Republicans have a narrow 51-49 majority, but this year’s map is decidedly unfavorable toward Democrats, with 10 of them running for re-election in states that President Donald Trump won in 2016.

In Tennessee, where Trump won by more than 25 points, a recent poll showed Bredesen ahead of Blackburn by 10 points.