The tea party group FreedomWorks PAC has taken the unusual step of withdrawing its endorsement of one candidate in a key GOP Senate primary and endorsing another.

The PAC had previously endorsed former state treasurer Shane Osborn. It will now back Midland University President Ben Sasse, who has gotten the support of several other tea party groups but whom FreedomWorks previously criticized as an Obamacare supporter.

In a statement, FreedomWorks head Matt Kibbe said both Osborn and Sasse are "great people," but that Osborn has allied himself with the GOP establishment.

“At this point, it is clear that Shane Osborn formed allegiances with Mitch McConnell and the K Street lobbying class," Kibbe said. "For us, that progression away from the grassroots has tipped the balance. FreedomWorks PAC has a responsibility to endorse the most reliable candidate for liberty, and after following the evolution of this primary, it’s clear that Ben Sasse is the man for the job.”

Just two months ago, the same group attacked Sasse as a supporter of many of the principles of Obamacare and said he profited from helping companies implement it.

"Mr. Sasse, a wealthy former business management consultant with a long list of health industry clients, is positioning himself as a staunchly 'anti-Obamacare' candidate; but that doesn’t square with his past record," said Dean Clancy, a now-former vice president with the group. (Clancy reportedly quit the group over the endorsement switch.)

The race appears to be a two-man contest, with the winner of the GOP primary heavily favored to succeed retiring Sen. Mike Johanns (R-Neb.).

The endorsement switch means Sasse has now pretty clearly locked up the tea party support in this race. He's also backed by the Senate Conservatives Fund and, more recently, former Alaska governor Sarah Palin.

Sasse appears ascendant in the race after early polls showed Osborn as the frontrunner. And last week, it was reported that Osborn's campaign provided an official-looking Navy memo to defend itself, but that the memo was actually written by a friend.

Osborn on Monday announced he raised $550,000 in the first quarter of the year. Sasse has yet to announce his fundraising totals.

Update 10:01 a.m.: Osborn's response to FreedomWorks's change of heart: “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.  Sasse is selling himself as the ObamaCare nemesis, but nothing could be further from the truth."