The tea party-aligned FreedomWorks has gone from tentative support for House Speaker John Boehner's "Plan B" to outright opposition. 

The group joins Club for Growth and the Heritage Foundation in opposing the GOP proposal, which would let  tax cuts on income over $1 million expire. House Republicans are planning a vote on the plan Thursday. 

In a blog post Wednesday, FreedomWorks's Dean Clancy celebrated Boehner's proposal as "good news!" 

"While not nearly as good as the FreedomWorks Plan to avert the fiscal cliff, Plan B is much better than the so-called 'balanced approach' that Mr. Boehner had, until Monday, been trying to negotiate with Mr. Obama," Clancy wrote. 

An update to that post Thursday afternoon shows that the group no longer sees the plan as an acceptable alternative. 

"After review of the Boehner Plan B legislation, pending in the House today, FreedomWorks has found it must oppose the legislation, and will be urging House members to vote NO on the bill," the statement reads. "We will post our formal opposition letter on our site, soon." (The headline still reads "Two Cheers for Boehner's Plan B.")

House Majority Leader Eric Cantor (R-Va.) has predicted that the plan will pass, regardless of conservative opposition. Senate Majority Leader Harry Reid (D-Nev.) has labeled the proposal a distraction from bipartisan negotiations and vowed that the upper chamber will not vote on it.

Republicans charge that President Obama has not put forward a balanced offer for the GOP to consider, a claim the White House strongly disputes.