WASHINGTON (CNN) -- An emotional Karl Rove characterized his tenure in the White House as a "witness to history" as he announced his resignation as President Bush's senior political adviser at the end of this month.

Rove and President Bush embrace after Rove announced his resignation.

"It seems the right time to start thinking about the next chapter in our family's life," Rove said, his voice breaking. "It's not been an easy decision."

Speaking to reporters outside the White House with Bush at his side, Rove said discussions about his departure began last summer.

"It always seemed there was a better time to leave, somewhere out there in the future. But now is the time."

He told Bush he would be "your fierce and committed advocate on the outside."

Bush said Rove "is movin' on down the road," and added, "I'll be on the road behind you here in a little bit."

The two men have known each other for decades. Rove said they have been friends for 34 years. Watch how Rove might influence the 2008 presidential election »

Rove, who has held a top position in the White House since Bush took office in January 2001, is to stand down on August 31.

Rove said earlier Monday that his resignation was not forced and that he plans to spend his post-White House career writing a book and teaching.

Perhaps Bush's most powerful White House aide, Rove submitted his resignation to Bush on Friday, he told CNN's Suzanne Malveaux in an e-mail.

Told that there are some who say he's being "run out of town," Rove responded, "That sounds like the rooster claiming to have called up the sun."

Though Rove has been praised as a political mastermind, that image was tarnished last year, when he insisted Republicans would hold onto both houses of Congress in the midterm elections. Democrats won control of both chambers.

And despite successes on Bush's domestic agenda, Rove failed to engineer a winning strategy on immigration reform after the midterm elections.

Rove has been subpoenaed to testify before the Senate Judiciary Committee investigating the firings of some U.S. attorneys, but the White House said Rove, as an "immediate adviser" to the president, cannot be ordered to testify.

Sen. Patrick Leahy, the committee's chairman, issued a statement following Rove's announcement saying Rove has "acted as if he was above the law."

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

Rove did testify before a federal grand jury about the leak of CIA operative Valerie Plame's identity after Plame's husband, former ambassador Joe Wilson, became a critic of the war in Iraq.

I. Lewis "Scooter" Libby, who was Vice President Dick Cheney's chief of staff, was later convicted of lying and obstructing justice. Bush commuted Libby's sentence.

Libby's attorneys contended that Libby was the victim of a White House conspiracy to protect Rove but never presented any evidence to support that claim.

Journalist Robert Novak, who identified Plame in a 2003 column, testified that Rove was one of two officials who leaked Plame's identity to him, but Rove was never charged with a crime.

His departure comes as Bush continues to face low approval ratings and as Republican presidential hopefuls work to differentiate themselves from Bush.

The resignation sparked speculation over whether Rove will soon join the campaign of a GOP candidate. But a Wall Street Journal article on Monday in which news of Rove's pending resignation came out said Rove will "offer advice, if asked, but at 56 years old says he is done with political consulting."

Other White House officials who left after the election include White House counselor Dan Bartlett, budget director Rob Portman, chief White House attorney Harriet Miers, political director Sara Taylor and deputy national security adviser J.D. Crouch. Defense Secretary Donald Rumsfeld left his job immediately following the election, as the Iraq war's influence on voters became clear.

Senior administration officials said White House Chief of Staff Josh Bolten recently told senior aides if you're staying after Labor Day, plan to stay for the remainder of the term, so make decisions now, which forced Rove to make his decision.

After the announcement, Bush and Rove were headed to the president's ranch in Crawford, Texas. Rove intends to return to Washington over the weekend, according to a White House official.

Rove said the first thing he plans to do after leaving the White House is "go dove hunting in West Texas with family and friends, then drive my wife and the dogs to the beach."

A senior administration official described Rove's agony over the decision, and how "he and his family struggled" over it and why "this is as good a time as any."

"You're never going to replace him," said another senior administration official, adding that Rove served a "unique role."

"It's up to [White House Chief of Staff] Josh [Bolten] whether he'll be replaced," the official said.

Rove plans to write a book about his days with Bush and eventually teach politics on the university level.

"Obviously it's a big loss to us," said deputy White House press secretary Dana Perino. "He is a great colleague, good friend and a brilliant mind."

Bush nicknamed Rove "the architect" for creating the plan that won the White House in 2000 and 2004. E-mail to a friend

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