The Club for Growth and the Independent Women’s Voice have asked to be removed from a letter blasting a proposal by House Ways and Means Chairman Dave Camp (R-Mich.) to pursue a miscellaneous tariff bill in possible violation of the House earmark ban.

The groups joined Americans for Tax Reform, which distanced itself from the document Wednesday, highlighting a rift among conservatives over the issue and raising questions about the integrity of the letter’s list of supporters.

The letter, organized by the Conservative Action Project, listed 26 conservative leaders as signatories and blasted Camp’s proposal as violating the House ban on earmarks.

At issue are about 1,300 duty suspension bills that Members of Congress have proposed for a miscellaneous tariff bill. Critics say the vast majority of the proposals are clear violations of the House ban on earmarks, while supporters say they do not violate the rules.

Referring to the debate, the memo says that “an earmark is an earmark is an earmark” and that the Camp proposal would invite corruption.

Grover Norquist, the president of Americans for Tax Reform, said Wednesday that he was misled about what the content of the letter would include.

“I was told this was a letter arguing against restoring spending earmarks,” Norquist said.

A spokesman for the CAP said: “All organizations represented signed on to that memo. If one organization has a second thought later, that’s understandable when you have 27 organizations signing on.”

Norquist and some other conservatives support Camp’s proposal because the duty suspensions would lower taxes on the companies importing products in each category in which the duties are suspended.

Other conservatives, such as the group Heritage Action for America, argue the process, in which individual companies petition lawmakers for changes to the law that will help those companies’ bottom line, is susceptible to corruption.