In an unusual move, FreedomWorks announced Friday afternoon that it was rescinding its endorsement of former Nebraska Treasurer Shane Osborn in the Republican Senate primary, and will instead support another primary contender, Midland University President Ben Sasse.

Matt Kibbe, the organization's president, said in a statement that it chose to change camps because Osborn “formed allegiances with Mitch McConnell and the K Street lobbying class.”

In response, Osborn's campaign said Sasse "has garnered the support of Washington, D.C. special interest groups because of his long history living and working in the city.

“FreedomWorks has decided to endorse a candidate who they have attacked for months over his lack of conservatism and record of championing big government policies," Osborn said in a statement. "Sasse is selling himself as the Obamacare nemesis, but nothing could be further from the truth."

A Nebraska Republican leader who’s been involved in state politics for decades told the Washington Examiner that FreedomWorks’ calculations are probably more complicated than that.

“The Osborn campaign has imploded in recent weeks,” said the insider.

Three factors converged to make Osborn an unappealing candidate, he continued. First, a friend of Osborn's at the Pentagon produced an unsigned memo for him, which he then gave to a reporter, according to the Omaha World-Herald newspaper. When news broke that the memo was unauthorized, Osborn said he took "full responsibility" for the situation, and described it as "regrettable." The friend who produced the memo has remained anonymous, and producing it could end his career in the military and potentially lead to criminal charges.

On top of that, Osborn has struggled to handle allegations that he personally profited from the federal bank bailouts, known as the Troubled Asset Relief Program. And, per the source, Sasse is receiving substantial grassroots support.

“Grassroots is coming out for Sasse like I’ve never seen in a couple decades,” he said.

“You add all those things up and what you see is one campaign in turmoil and another campaign that clearly has momentum and support,” he added.

Earlier Friday afternoon, FreedomWorks still had a blog post on its website written by former Vice President of Public Policy Dean Clancy charging that Sasse “supports the basic principles of Obamacare.”

“My sense is Ben Sasse is not what he claims to be,” Clancy continued. “I think he's a Mitt Romney-style technocrat masquerading as an anti-Obamacare firebrand for temporary political purposes. Having devoted a good part of my adult life to fighting not just the actual realities but the underlying principles of Obamacare, I find that appalling.”

UPDATE: This story was updated at 11 p.m. March 28 to include comments by the Osborn campaign, and at 6:09 p.m. March 29 to clarify that Dean Clancy no longer works at FreedomWorks. It was updated at 10:20 p.m. March 30 to include more details about the letter controversy. It was originally posted at 6:28 p.m. March 28.