Grover Norquist, head of Americans for Tax Reform, says letting lower tax rates expire isn't the same as imposing new taxes. (Rich Clement/Bloomberg)

The powerful anti-tax advocacy group Americans for Tax Reform said Wednesday that it would not consider a vote for House Speaker John Boehner's "Plan B"  a violation of the Taxpayer Protection Pledge many Republican members of Congress have signed. 

"Republicans supporting this bill are this week affirming to their constituents in writing that this bill—the sole purpose of which is to prevent tax increases—is consistent with the pledge they made to them," ATR said in a statement.  "In ATR’s analysis, it is extremely difficult—if not impossible—to fault these Republicans’ assertion." 

ATR leader Grover Norquist has dogged members of the House and Senate who are willing to violate the "no new taxes" pledge as part of a "fiscal cliff" deal. But Norquist himself has said in the past that allowing tax cuts to expire is not the same as supporting a tax increase. Norquist said at the time that he was misinterpreted and that the expiration of the lower rates would be a tax increase, but he did not call it a pledge violation. 

The statement Wednesday makes clear that, at least in this case, letting tax cuts expire does not violate the pledge. 

"The House has already voted twice to prevent any tax increases on any American," the statement reads. "When viewed with this in mind, and considering this tax bill contains no tax increases of any kind—in fact, it permanently prevents them—matters become more clear." 

Boehner's 'Plan B' bill would allow tax cuts to expire for those making more than $1 million. But its fate is unclear. Some conservatives are still wary of letting rates rise at all. The Club for Growth and the Heritage Foundation have come out against it. And the White House has promised to veto the legislation should it pass, saying it helps millionaires at the expense of the middle class.