The anti-tax group Club for Growth is warning lawmakers not to vote for House Speaker John A. Boehner’s proposal to raise taxes on Americans making more than $1 million annually, because the bill is “a bargaining tactic.”

“The bill is anti-growth,” the group’s vice president of government affairs, Andy Roth, wrote in a letter sent to Capitol Hill Wednesday, adding that the group will score how lawmakers vote on the bill.

Boehner’s proposal “increases tax rates for those making over $1 million while also raising taxes on capital gains and dividends. We don’t buy into the Washington-speak, suggesting that these are actually tax cuts. Also, it’s no secret that this bill is not a final product, but a bargaining tactic to make a larger deal even worse. As such, we cannot support it just on substance, but also the procedure which dictates its consideration. Therefore, we are urging all members to oppose the rule," Roth wrote.

Americans for Tax Reform, another anti-tax group led by activist Grover Norquist, said earlier Wednesday that the Boehner proposal does not violate a pledge signed by many Republicans to never raise taxes. 

In a further splinter of conservative groups, Heritage Action for America was also urging House Republicans to vote against Boehner's plan.

Passage remains uncertain. Republicans can likely lose only 17 votes in the House, and a number of conservative members have already spoken out against the proposal.