Top White House aide Karl Rove, seen by many as the brains behind George W Bush's presidency, has said he will resign at the end of August.
Karl Rove has worked with Mr Bush since 1993
"I just think it's time," Mr Rove said in an interview with the Wall Street Journal, adding that he was quitting for the sake of his family.
Mr Rove has worked with Mr Bush since 1993 when he ran for Texas governor.
As Mr Bush's chief strategist, he is seen as instrumental in delivering election victories in 2000 and 2004.
For this he is highly regarded by Republicans, but at the same time equally reviled by Democrats.
"Obviously, it's a big loss to us," White House deputy press secretary Dana Perino told the Associated Press news agency. "He's a great colleague, a good friend, and a brilliant mind. He will be greatly missed."
"He will continue to be one of the president's greatest friends," she added.
Mr Rove has been accused of underhand political tactics since his teenage years.
As a student, he invited Chicago vagrants to turn up for free beer at a plush reception for a Democrat state candidate - an incident he later described as a "youthful prank" that he regretted.
He has continued to be dogged by controversy.
Last month, the US Senate issued a subpoena against him as part of an investigation into the sacking of eight federal prosecutors, but Mr Bush ordered him not to testify, citing executive privilege.
ALL THE PRESIDENT'S MEN
Karl Rove, chief political strategist, leaves 31 Aug
Donald Rumsfeld, defence secretary, quit after 2006 mid-term elections
Andrew Card, chief of staff, quit March 2006
Paul Wolfowitz, deputy defence secretary, quit March 2005
Colin Powell, secretary of state, resigned after President Bush's first term, Nov 2004
SURVIVORS FROM 2000
Dick Cheney, vice-president
Condoleezza Rice, secretary of state, was national security adviser in first term
Alberto Gonzales, attorney general, was White House counsel
Mr Rove was also investigated in connection with the exposure of CIA agent Valerie Plame, though prosecutors decided he should not face any charges.
Mr Rove told the Wall Street Journal that he had first floated the idea of leaving last year, but had delayed his departure when the Democrats took control of Congress.
He said he took a final decision to leave after White House Chief of Staff Joshua Bolten told aides that if they stayed after the end of August they would be obliged to stay in the administration until Mr Bush's own departure in January 2009.
"There's always something that can keep you here, and as much as I'd like to be here, I've got to do this for the sake of my family," he said.
He said he expected Mr Bush's current poor ratings to improve, and that conditions in Iraq would get better as the military surge continued.
A Republican had a good chance of winning the 2008 presidential election, he said, because Democrats would choose the "fatally flawed" Hillary Clinton as their candidate.