He said he would "consider use of compulsory process" if the Treasury Department does not provide more information by Aug. 29.
IRS plans call for making subsidies available in all 50 states, but some congressional Republicans argue the tax credits should not be available in states that operate their own insurance exchanges under ACA. Issa believes the Treasury and IRS plan for implementation of the subsidies is illegal, The Hill said.
In his letter to Lew, Issa said documents the department has turned over to his committee do not fully explain the decision to offer subsidies in all 50 states.
Treasury and the IRS have said Congress' intention was clear -- that there should be federally operated exchanges in states that decide not to operate their own exchanges -- The Hill said.