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Mr. Rove said he first mentioned resigning to the president a year ago, but stayed on after Democrats took control of Congress to help Mr. Bush with the Iraq war and immigration debates. He blamed the Republican loss of the House and Senate on personal scandals involving Republican lawmakers.

Mr. Rove said that Chief of Staff Joshua B. Bolten told White House officials that if they stayed past Labor Day, they would be expected to remain for the rest of Mr. Bush’s term. No replacement has been named, though current White House counselor Ed Gillespie’s name has figured in speculation.

One former Bush administration official said that Mr. Rove is leaving to focus all his energies on shaping Mr. Bush’s legacy, partly through building the presidential library at Southern Methodist University in Dallas.

“I think he’s got the legacy mission now,” the former administration official said. “He wants to make sure that president’s place in history is appropriately shaped.”

Mr. Rove said he would continue to be Mr. Bush’s “fierce and committed advocate on the outside.” He plans to write a book, with Mr. Bush’s encouragement, about his time in the White House.

Several Democrats said the departure was intended to frustrate congressional probes of the U.S. attorney firings. However, Senate Judiciary Committee Chairman Patrick J. Leahy, Vermont Democrat, said that his panel “will continue its investigation.”

“I’m realistic enough to understand that the subpoenas are going to keep flying my way,” Mr. Rove said aboard Air Force One. “I’m Moby Dick, and we’ve got three or four members of Congress who are trying to cast themselves in the part of Captain Ahab.”