Fran Townsend, the leading White House-based terrorism adviser who gave public updates on the extent of the threat to U.S. security, is stepping down after 4 1/2 years.

President Bush said in a statement Monday morning that Townsend, 45, "has ably guided the Homeland Security Council. She has played an integral role in the formation of the key strategies and policies my administration has used to combat terror and protect Americans."

Her departure continues an exodus of key Bush aides and confidants, with his two-term presidency in the final 15 months. Top aide Karl Rove, along with press secretary Tony Snow, Attorney General Alberto Gonzales, Defense Secretary Donald Rumsfeld and senior presidential adviser Dan Bartlett, have already left.

"Fran always has provided wise counsel on how best to protect the American people from the threat of terrorism," the president said in his statement. "She has been a steady leader in the effort to prevent and disrupt attacks and to better respond to natural disasters."

In her handwritten resignation letter to Bush, Townsend wrote, "It is with a profound sense of gratitude that I have decided to take a respite from public service." White House press secretary Dana Perino said Towsend struggled with the decision, talking about it with the president for months.

Townsend has told colleagues she is looking for opportunities in the private sector. For someone who at one point had figured in speculation as to who would head the then-new Department of Homeland Security or assume the newly created post of national intelligence director, she became a familiar face for the administration, often appearing on morning news and Sunday interview shows to present Bush's case.

When Democratic Sen. Russ Feingold called on Bush to refrain from using the phrase "Islamic fascists" on grounds it was offensive to Muslims, Townsend explained the president's use of the phrase.

"What the president was trying to capture was this idea of using violence to achieve ideological ends — and that's wrong," Townsend said at a news conference. "Regardless of what label you pin on it, it is this form of radical extremism that really wants to deny people freedom and impose a totalitarian vision of society on everyone, that we object to."

She said the White House response to the devastating wildfires in California were going "exactly the way it should be" and assuring Californians that federal performance would be "better and faster" than after Hurricane Katrina's strike against the Gulf Coast states in 2005.

"This is not the end of federal assistance. It's just the beginning," Townsend said in connection with the wild fires.

Bush noted in his statement that Townsend prosecuted violent crimes, narcotics offenses, Mafia cases and white-collar fraud as an assistant district attorney in Brooklyn, N.Y. and as an assistant U.S. attorney in Manhattan.

There was no word on a successor for Townsend. Perino said officials intended to act "relatively soon," because Bush wants some overlap between Townsend and her replacement before she leaves just after the first of the year.

Bush has seen a substantial revamping of the lineup of players on the team he brought to Washington as the just-elected president in a disputed election with Democrat Al Gore in 2000.

He saw longtime friend, aide and confidant Gonzales resigned earlier this fall in the face of a convulsive uproar on Capitol Hill over the dismissals of a slew of federal prosecutors and in connection with the administration's warrantless wiretap program. And Rumsfeld resigned just after the time of the 2006 elections in which Democrats, harping on a get-out-of-Iraq theme, regained control of Congress.

Perino shrugged off the notion that a loss of top talent will hurt Bush's last months in office, noting the recent recruitment of experienced hands such as new Attorney General Michael Mukasey and White House counselor Ed Gillespie.