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.
"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.
"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"
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.
Delayed departure
Mr Rove has been accused of underhand political tactics since his teenage years.
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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.
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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.
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