A key member of President Bush's inner circle stepped down Thursday — the second administration shake-up in 24 hours — when Harriet Miers turned in her resignation as chief counsel.

White House Press Secretary Tony Snow said the president reluctantly accepted her resignation, which takes effect Jan. 31. Miers said she planned to return to the private sector.

Miers' resignation follows news that National Intelligence Director John Negroponte will step down from his post to become deputy secretary of state. In April 2005, Negroponte became the nation's first manager of all 16 intelligence agencies the U.S. operates.

A replacement for Miers has yet to be named. A top candidate for the intelligence chief opening is retired Vice Adm. Mike McConnell, the director of the National Security Agency from 1992 to 1996 who spent more than 25 years as an intelligence operations and security officer. McConnell is now a senior vice president at Booz Allen Hamilton, a government contractor and consulting firm.

Negroponte, 67, has held a series of tough posts in the Bush administration and has been at the center of the Iraq debate since before the U.S.-led invasion in March 2003. He served as U.S. ambassador to the U.N. from 2001 to 2004 and ambassador to Baghdad until March 2005 before becoming intelligence chief.

Current and former Bush administration officials are alternatively portraying the move as a reflection of Negroponte's preference for diplomacy over intelligence work, his frustration in his current job and a political blunder that will open up an unnecessary fourth confirmation hearing at a time when three others are scheduled to be held by the Democrat-controlled Senate.

Miers grabbed headlines in October 2005 when Bush nominated her to replace retiring Justice Sandra Day O'Connor. Miers was later forced to withdraw her nomination after conservatives questioned her qualifications.

Snow offered no particular reason for her departure, and said she made her decision on Wednesday.

"Basically, she has been here six years," Snow said. "As somebody said earlier today 'She put 12 years of service into six years.'"

He dismissed questions that Miers' resignation was part of a White House shake-up.

Asked if other officials were poised to go, Snow said, "I'm aware of none and anticipate none."

But a well-placed source told FOX News that Bush adviser Karl Rove has wanted for some time to establish an independent unit in the White House to deal exclusively with the far-reaching investigations the Democrat-controlled committees in the House and Senate are expected to launch this year. Miers resignation is linked to that wish.

Since her entire career has been spent in civil litigation, Miers was considered not versed enough in criminal law and separation-of-powers issues to lead the counsel's office during a period expect to be dominated by Democrat-led investigations, the source explained. The next counsel is accordingly expected to have a strong background in criminal law, federal prosecutions and the like.

According to Snow, Miers, a loyal adviser to the president for years, held several conversations with white House chief of staff Joshua Bolten about leaving. Both agreed that it was time for a change at the White House office of legal counsel.

"Harriet is one of the most beloved people here at the White House," Snow said, adding that she was a scrupulous lawyer who aggressively defended the Constitution.

As White House counsel, Miers works behind-the-scenes overseeing a team of attorneys who provide legal advice to Bush on matters large and small. In her resignation letter, she noted her role in vetting candidates for the job of federal judges.

"Participating in the process to help identify the best nominees for the American people has been among the most rewarding of my experiences," Miers wrote in the letter dated Thursday. "Your commitment to nominating judges who will interpret the law and who know the proper role of a judge has made this nation stronger and our justice system fairer."

Click here to read Miers' resignation letter.

Miers, 61, grew up in Dallas and received her undergraduate and law degrees from Southern Methodist University. She was the first woman hired by her law firm, in 1972; first woman president of the Dallas Bar Association, in 1985; first woman president of the Texas bar, in 1992; and first woman president of her law firm, in 1996.

The soft-spoken Miers has described herself as a "Texan through and through." Rove has said she can be "tough as nails." Bush once called her "a pit bull in size 6 shoes."

She has clerked for a federal judge and joined Locke Purnell Rain Harrell, rising to become its first female president. After the firm merged with another, she was co-managing partner of the 400-lawyer Locke Liddell & Sapp.

She was Bush's personal lawyer in Texas, took on the thankless job of cleaning up the Texas Lottery when he was governor, and followed him to Washington to serve as staff secretary until Bush appointed her White House counsel, succeeding Alberto Gonzales when he was named attorney general.

Negroponte 'Not Doing Well' as DNI

Negroponte is moving to a job that is on paper junior in rank to the one he now holds. One official told FOX News that Negroponte is more comfortable in the State Department setting.

"He's a foreign service officer to the bone," the official said, adding, "Everyone knows he's not doing well (as DNI). If you ask the people in the intelligence community, he was a disaster. The best he did was to turn the [intelligence community] over to the care and mercy of the foreign service. ... Nothing's gone on since he got there but another layer of bureaucracy. He did nothing to revive the intelligence agencies."

The official said that nothing has been done to suggest the intelligence community is better off since former CIA Director George Tenet left.

Negroponte is viewed with suspicion by conservatives, who regard him as soft on weapons of mass destruction non-proliferation issues.

"He is a paper tiger," said a former administration official still frequently consulted by the White House, quoting Republican senators. "What control does he have? None."

Sen. John Rockefeller, D-W. Va., chairman of the Senate Select Committee on Intelligence, will oversee confirmation hearings for Negroponte's replacement.

Rockefeller said he is concerned about the vacancies of both director and deputy national intelligence director.

"I am deeply troubled by the timing of this announcement and the void of leadership at the top of our intelligence community," Rockefeller said in a statement.

"It is not acceptable for the top two jobs to be vacant at the same time. The leadership of the Intelligence Community is too important."

In a time of "tremendous tension" between the nation's intelligence agencies, this source said Negroponte found himself unable to bend them to his will and suggested that Negroponte accepted the job at the State Department because he did not want to preside over the intelligence agencies this spring when Senate Democrats are expected to approve legislation tightening congressional oversight of the National Security Agency's controversial warrantless wiretapping program.

"If the [expletive] is going to hit the fan," the former official said, "maybe [Negroponte] wants to not be there when it happens."

The source added that the Negroponte nomination "opens up a can of worms for the administration … on wiretaps."

A palpable fear is also evident among those concerned that Negroponte will "clean house" at the State Department and replace officials dedicated to the implementation of Bush-Cheney foreign policy with career foreign service officers harboring a different agenda.

"We're in for some rough waters," the individual said.

The Democrat-controlled Senate will now be required to hold confirmation hearings for four senior positions — deputy secretary of state, director of national intelligence, deputy director of national intelligence and U.N. ambassador. John Bolton left that post last month after failing to win Democratic support for a permanent placement in the post. The job held by Miers does not require Senate confirmation.

All the confirmation hearings will follow shortly after the president gives his address on a new strategy for Iraq and delivers his State of the Union speech.

Robert Zoellick resigned as Secretary of State Condoleezza Rice's deputy in July to take a position with the Wall Street firm Goldman Sachs. As for why Rice would select Negroponte as deputy secretary, various sources suggested others had been approached for the job, including Under Secretary Nicholas Burns, and turned it down.

"My sense is that the wagons are being circled tighter," said the former official. "Condi, like the president, feels comfortable with people she knows and has worked with and wouldn't have felt comfortable with a deputy she didn't really know."

A third source, a current administration official, said people are "reading too much" into the Negroponte shift. "He's a career diplomat and wanted to go back there," this source said.

FOX News' Bret Baier and James Rosen and The Associated Press contributed to this report.