Some Republican members of the Senate Finance Committee wrote to the head of the IRS on Wednesday to request information about the agency’s enforcement of a historically ignored gift tax on contributions to the type of nonprofit advocacy groups that are poised to play a major fundraising role in the 2012 elections.

In the letter, which was prompted by media reports that the IRS has recently begun to review past donations to 501(c)(4) organizations, the Senators question whether the enforcement tactic is politically motivated, noting the “deep hostility” for the groups that has been expressed by the White House.

“It is not only imperative that taxpayers are able to rely on consistent and foreseeable enforcement action from the IRS,” the Senators wrote. “It is also essential that citizens have confidence that those decisions are never influenced by political considerations.”

Sens. Orrin Hatch (Utah), Jon Kyl (Ariz.), Pat Roberts (Kan.), John Cornyn (Texas), John Thune (S.D.) and Richard Burr (N.C.) requested the names of the IRS employees involved in the decision to review 501(c)(4) contributions, as well as related correspondence between the agency and the executive branch, from Commissioner Douglas H. Shulman by May 31.

For decades, the IRS has done little to enforce a statute that requires individuals to pay a tax on money given to organizations that are tax-exempt but not charitable in mission. But there are at least a half-dozen reports of individuals who have recently received a letter from the estates and gifts section of the IRS informing them that their 501(c)(4) contributions are under review, potentially creating large gift tax liabilities for donors who wrote big checks to these groups during the previous election.

“This pattern of nonenforcement over a period of nearly three decades, coupled with the troubling issues regarding the adverse impact that enforcement might have on the exercise of constitutionally protected rights, raises important questions regarding the timing of the decision to enforce the gift tax on these contributions,” the Senators wrote.