Sen. Bob Corker couldn't keep a straight face while 'endorsing' Rep. Marsha Blackburn.

Republicans have pinned their hopes for retaining the Senate on a candidate so extreme, her own home-state colleague couldn’t get through an endorsement without laughing. Instead, Sen. Bob Corker could only stammer and laugh his way through a tepid response.

Tennessee Rep. Marsha Blackburn is running to replace the retiring Corker. But she’s facing an uphill fight against surging former Democratic Gov. Phil Bredesen, who’s receiving strong support from Republican voters.

On CNN’s “State of the Union” Sunday morning, Blackburn was dealt another blow. Host Dana Bash asked Corker why Blackburn should replace him. He couldn’t manage to come up with a real answer.

“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,” Corker said.

“That’s not a ringing endorsement of Marsha Blackburn, to say she should be elected because she will vote for Mitch McConnell,” Bash pointed out.

Corker fell silent, then stammered and even laughed softly. “I’m supporting the nominee. I have worked with the nominee for some time,” he finally replied.

“I don’t know what else to say.”

Corker’s “endorsement” speaks volumes about Blackburn’s extremism.

When Blackburn announced her candidacy, Twitter actually took it down because it was rife with lies and offensive statements about Planned Parenthood. She is equally extreme on issues like guns, which she compared to “hammers and hatchets.” And has promoted the racist “birther” conspiracy theory about President Barack Obama.

Blackburn is yet another Republican at serious risk of defeat in the midterms, even in a state that Trump won by 26 points in 2016. Trump has clearly been hurting Republicans in special elections. But putting up terrible candidates like Blackburn can’t be helping, either.