"I'm grateful to have been witness to history," said an emotional Rove, dubbed "Bush's brain" by critics, as he stood side-by-side with the president to make his resignation as deputy White House chief of staff official.

"I've seen a man of farsighted courage put America on a war footing and protect us against a brutal enemy in a dangerous conflict that will shape this new century," Rove said.

Rove, a scourge of Democrats, rocked Washington's political establishment when he first divulged his plans in an interview in the editorial pages of the Wall Street Journal. Bush praised Rove, who will leave at the end of the month, for making "enormous sacrifices" to serve, and in a nod to his own waning days in power said: "I will be on the road behind you here in a little bit."

But Democrats, who have repeatedly been the victims of Rove's hard knuckle brand of politics, said he had left a stain on Washington.

Senate Judiciary Committee chairman Patrick Leahy, who has subpoenaed Rove in a row over federal prosecutors Democrats say were fired for political gain, accused him of manipulating elections and putting himself above the law.

"There is a cloud over this White House, and a gathering storm. A similar cloud envelopes Mr. Rove, even as he leaves the White House."

Rove, who enjoys near mythical status in Republican circles, had been expected by most observers to stay until Bush leaves office in January 2009.

There will now be speculation as to whether he will take a role in any of the 2008 campaigns of Republicans vying to succeed Bush in the White House.

"I've asked a lot of my family, and they've given all I've asked and more," Rove said.

"Now it seems the right time to start thinking about the next chapter in our family's life."

The timing of Rove's departure means he will absent from the White House's political battle with Congress expected in September when the top US general and diplomat in Iraq deliver a progress report on Bush's war strategy.

Rove's county-by-county knowledge of the US political map helped Bush win two close-fought elections, by appealing to conservatives and disdaining the conventional wisdom that US elections are won from the center.

But he leaves office having failed in his ultimate political ambition, of piecing together an enduring conservative ruling majority, after Democrats seized control of Congress last November.

Bush, who once dominated the US political scene, is mired in a prolonged slump: the president's job disapproval ratings stood at a dismal 63 percent at the end of last month, the worst showing since president Jimmy Carter, sending alarm bells through the Republican establishment.

An astute operator from Texas who has been with Bush since his gubernatorial campaigns of the 1990s, Rove has been under fire since 2003, when retired US diplomat Joseph Wilson claimed he had illegally leaked to the media the identity of Wilson's wife, Valerie Plame, a covert CIA employee.

Wilson asserted the leak had been orchestrated in retaliation for his questioning the Bush administration's rationale for war in Iraq. Rove was never charged but the probe led to the conviction of Lewis "Scooter" Libby, a former chief of staff for Vice President Richard Cheney.

Rove has been widely reported to have played a key behind-the-scenes role in persuading Congress to endorse the March 2003 invasion of Iraq.

"Iraq will be in a better place," Rove said in the Journal interview, despite criticism that the troop "surge" strategy announced by the president at the beginning of the year was not bearing any fruit.

He also made headlines in 2005, arguing that following the September 11, 2001 terrorist attacks, liberals wanted to "offer therapy and understanding for our attackers" instead of tough military action.