WASHINGTON, Aug. 13 — Karl Rove, the political adviser who masterminded President George W. Bush’s two winning presidential campaigns and secured his own place in history as a political strategist with extraordinary influence within the White House, announced on Monday that he would resign at the end of the month.

In an unusually emotional appearance with President Bush on the South Lawn of the White House, Mr. Rove cited a desire to “start thinking about the next chapter in our family’s life.” His decision was also forced when the White House chief of staff, Joshua B. Bolten, recently told senior aides that if they stayed past Labor Day he would expect them to stay through the remaining 17 months of Mr. Bush’s term.

Mr. Rove and Mr. Bush, who said they had known “each other as youngsters” interested in politics, first discussed his departure last summer, Mr. Rove said, his voice breaking at times. Instead he stayed on, through the midterm elections last fall, which put Democrats in control of Congress and tempered Mr. Rove’s reputation as a political genius who had ushered in an enduring Republican majority.

“It always seemed there was a better time to leave out there in the future,” Mr. Rove said, “but now is the time.”