The Senate Indian Affairs Committee is seeking documents from Americans for Tax Reform as part of its inquiry.
It's looking into allegations that lobbyists Jack Abramoff and Michael Scanlon defrauded tribal clients, including the Mississippi Band of Choctaw Indians, of millions of dollars while representing them on casino issues.
Americans for Tax Reform is an advocacy group headed by conservative political activist Grover Norquist. It has refused to hand over donor records.
Spokesman Christopher Butler says the contributors' identities are confidential and that the committee won't get them unless it agrees to keep them secret.
In a written statement, he says the group wants the committee to go to the tribes to find out whether they donated to the group. He says the group already has provided records that don't identify donors.