If Republicans lose the Senate in November, Tennessee may be the state that does them in. It's arguably their third most vulnerable seat, and they can lose only a net of two seats.

And if it is the difference-maker, Sunday may be a day they look back upon with frustration.

Appearing on CNN, outgoing Sen. Bob Corker (R-Tenn.) was asked about a Washington Post report that GOP campaign officials were upset he was saying too many nice things about the Democrat seeking to succeed him, former governor Phil Bredesen. Corker expressed exasperation about even being asked the question ... but then he proceeded to show just how legitimate it was.

Corker wouldn't even say the name of the Republican seeking to replace him, Rep. Marsha Blackburn, and when pressed, he couldn't even find something nice to say about her personally. All he could muster was that she would vote for a Republican to lead the Senate — which is a pretty low bar.

Here's the exchange:

DANA BASH: Can you tell me why Republican Marsha Blackburn is better to represent your state in Tennessee and take your seat than Phil Bredesen?

CORKER: Well, I think most people in our state — it is a red state — will focus on the first vote she makes. And that is the vote to elect the majority leader. And I think, at the end of the day, that is going to be a big factor in the race. But, Dana, the rest of the story wasn't written during the interview I had with “The Christian Science Monitor.” And, somehow or another —

[CROSSTALK]

BASH: Senator, that's not a ringing endorsement of Marsha Blackburn, to say that she should be elected just because she is going to vote for Mitch McConnell.

[AWKWARD PAUSE]

CORKER: Well, Dana, you know, I'm supporting the nominee. I have worked with the nominee for some time. And I don't know what else to say.

BASH: Okay, we will leave it there.

“I don't know what else to say.” Two chances to say something nice about Blackburn, two hard passes.

Corker defended himself just before the exchange above by noting he had given the maximum contribution to Blackburn's campaign. But this is so much more valuable to Bredesen than $5,400. This wasn't just Corker keeping his powder dry because he's friends with Bredesen; this was Corker making clear that he's also not a huge fan of Blackburn's. And the fact that Corker couldn't find anything nice to say about Blackburn is a stark contrast to his having called Bredesen “a very good mayor, a very good governor, a very good business person” just last week.

It's perhaps understandable that there would be tension between Corker and Blackburn. Blackburn's campaign lashed out when Corker was reconsidering his decision to retire a couple months back, decrying “ego-driven, tired old men” and saying claims she might lose the seat were “sexist.”

But the tortured responses from Corker seem to be particularly valuable to Bredesen given how this race is shaping up. Bredesen was a very popular governor a while back, and the race is polling competitively. Yet Tennessee is a red state, and he's going to get hammered for his party affiliation. If conservative-leaning voters are going to vote for the blue side, they need to be convinced that Bredesen isn't going to be a down-the-line Democrat and that it is worth crossing over for him.

A vote of confidence from the outgoing GOP senator should go a long way in making them believe that Bredesen would be independent, and now Corker is going a step further by suggesting that Blackburn isn't particularly great, either.

No wonder GOP officials are pulling their hair out over this.