President George W. Bush last night rejected a subpoena for adviser Karl Rove to appear before to a Senate committee in an inquiry into the firing of federal prosecutors.
The move by the president, who cited executive privilege, sets up a possible court showdown between the White House and Democratic lawmakers, who have sought to force Mr
Rove and other Bush aides to testify about the firing last year of nine federal prosecutors.
Critics say they were fired for political reasons.
"Mr Rove, as an immediate presidential adviser, is immune from compelled congressional testimony about matters that arose during his tenure and that relate to his official duties in that capacity," White House Counsel Fred Fielding wrote in a letter to Senate Judiciary chairman Patrick Leahy, a Vermont Democrat.
"It is a shame that this White House continues to act as if it is above the law. That is wrong," Mr Leahy said in a statement.
"The subpoenas authorised by this Committee in connection with its investigation into the mass firings of US attorneys and the corrosion of federal law enforcement by White House political influence deserve respect and compliance."
The committee had subpoenaed Mr Rove to testify at a hearing this morning, along with another White House aide, deputy political director Scott Jennings.
Mr Rove will not appear at the hearing, while Mr Jennings will appear but not testify about the fired prosecutors.
Democrats say the firings may have been intended to influence investigations of Democratic or Republican lawmakers.
Mr Bush and Attorney General Alberto Gonzales, who signed off on the firings, have said that they were justified but mishandled.
Mr Gonzales also faces calls for a perjury investigation over the truthfulness of his testimony to Congress about the firings and the National Security Agency's warrantless domestic-spying programme.
With the support of the president, Mr Gonzales has rejected bipartisan calls to resign.