The increasingly contentious Republican primary for U.S. Senate in Mississippi got even testier on Wednesday when state Sen. Chris McDaniel sent an open letter to Sen. Thad Cochran accusing his campaign of "shameful slander" and challenging him to debate during the final 13 days of the race, something the Cochran camp appears to have no interest in doing.

Republican primary challenger Chris McDaniel  (R). (AP Photo/Rogelio V. Solis)

"Senator, if you are inclined to cast aspersions on my honor and integrity then I call upon you to do it to my face in a debate forum," McDaniel wrote.

Until then, he added, "I will not engage either your campaign or the liberal media in their absurd witch hunt."

When asked about McDaniel's challenge, Cochran spokesman Jordan Russell suggested the campaign was not interested. He pointed to what the Cochran campaign says is a shifting version of events from the McDaniel camp in response to a blogger who was arrested last week for allegedly filming Cochran's bedridden wife at a nursing home.

"I think the internal debate inside the McDaniel campaign is ongoing," said Russell. "And I think that injecting a third party at this point wouldn't be very productive."

The blogger, who has praised McDaniel and criticized Cochran, was taken into police custody Friday on allegations he entered the nursing home where Cochran's wife has been staying for more than a decade to photograph her and post an image in an online video. Cochran's wife suffers from dementia.

The McDaniel camp maintains it had nothing to do with the video. But Cochran's campaign has raised questions about what McDaniel's staff knew and when. Meanwhile, the revelation that Cochran's campaign was aware of the video weeks before alerting authorities has raised questions from McDaniel.

The Fix has a good timeline of the entire episode.

It all comes as the campaign hits the stretch run. The primary is June 3. National tea party groups have lined up behind McDaniel. The race could represent one of their last best chances to score a victory in a contested Republican primary this cycle.