Sen. Bob Corker insists he's not retiring out of frustration with the slow pace of the Senate, but does admit that the Senate isn't exactly the right place for someone who wants to get something done.

"I'm in a very enviable position here as chairman of the Foreign Relations Committee," Corker told the Washington Examiner this week. "I'm not leaving out of frustration. I'm frustrated we're not getting more done, that has nothing to do with me leaving."

"A little, honestly. Yeah. You want to know the truth? Yeah ... I wouldn't want that to sound that it takes away anything from what I do," Corker said. "Am I burned out on hearings, hearing, hearing, hearings? Yeah, a little bit. I'm more of a doer. Yeah ... I hit the wall a little bit on that."

Corker's decision to leave sets up the possibility that conservative activists might try to fill his seat with someone to the right of the establishment, just as Judge Roy Moore is threatening to do in Alabama. Steve Bannon, the former White House chief strategist and current Breitbart News chairman, is spearheading the effort to get more conservatives up to Congress.

But at least one Senate leader says the GOP isn't worried about that.

"Looking at the list, I don't see that," said Sen. John Thune, R-S.D. "I don't think it's a trend."

Bannon took on a high-profile role in the Alabama runoff to promote Judge Moore, but he hasn't settled on a candidate in Tennessee yet. Some Trump allies hope Rep. Marsha Blackburn, R-Tenn., enters the race. She is currently evaluating the situation and could decide by next week. Meanwhile, Breitbart has published pieces friendly to State Sen. Mark Green in recent months.

Corker said Wednesday that he doesn't have a preferred candidate to become the Republican primary, although he said it's "very probable" Blackburn and former Rep. Stephen Fincher will jump in. He also said it's "very possible" Gov. Bill Haslam takes the plunge.

Thune and others said Corker was a loss for the Senate GOP.

"A big loss," said Thune. "He's always been a guy who's really about trying to find solutions, common ground, and getting results."

"I regret it," Sen. John McCain said. "He's one of the more outstanding individuals, and has done a great job as chairman of the Foreign Relations Committee."

"We're going to miss him, but none of us are indispensable," said Sen. John Cornyn, R-Texas. "I hope Tennessee sends us another good senator."