US President George W Bush has ordered close adviser Karl Rove not to testify before a Senate hearing on the sacking of eight federal prosecutors.
Karl Rove is accused of knowing about the sackings in advance
Mr Bush used the executive privilege he has as president to exempt Mr Rove from having to appear.
The US Senate committee is investigating whether the White House arranged the sackings for improper political reasons.
The Bush administration maintains that the dismissals were justified.
"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 lawyer Fred Fielding wrote in a letter to Democrat Senator Patrick Leahy, and made available to the Reuters news agency.
Mr Rove had been due to testify at a hearing on Thursday morning, along with another White House aide, deputy political director Scott Jennings.
Mr Jennings is still expected to appear but he is not expected testify about the fired prosecutors.
The row began after Attorney General Alberto Gonzales fired eight federal prosecutors in 2006 - an unusual move, but not illegal.
However, the controversy over the firings has grown into a larger dispute between Congress and the White House.
Democrats, who control Congress, say the sackings were politically motivated and that Mr Rove knew of discussions about firing the attorneys nearly two years before the axe fell on them.
Mr Gonzales has resisted repeated calls to resign
The president has previously offered to let Mr Rove and other aides speak privately to some members of Congress, but he has firmly rejected the demand for public testimony under oath.
Mr Rove and Mr Jennings were subpoenaed a day after the House judiciary committee issued contempt of Congress citations against White House chief of staff Joshua Bolten and ex-legal counsel Harriet Miers in the row.
The Democrats have also announced they will seek a perjury investigation to be carried out into Mr Gonzales in connection with the same case.
Opponents say he fired the attorneys for political reasons and later lied about the reason for their dismissal.
Mr Gonzales, who retains Mr Bush's support, says he did nothing wrong and has resisted calls for his dismissal.