WASHINGTON, Jan. 4 — Harriet E. Miers, the White House counsel who endured a brief moment in the spotlight as President Bush’s Supreme Court nominee in 2005, is leaving the administration, Mr. Bush’s chief spokesman said today.

Ms. Miers’s resignation will take effect Jan. 31, and a search for a successor is under way, the White House spokesman, Tony Snow, said. Mr. Snow said the resignation was not part of a big shift in the presidential staff.

“She’s been here six years,” Mr. Snow said. “It’s hard duty. Yes, it really is.” Ms. Miers was known for her marathon work days in the service of President Bush.

Mr. Snow added, “She is beloved not only because she is a really good human being, she’s an extraordinarily wonderful human being, but also somebody who is a very careful and scrupulous lawyer, a ferocious defender of the Constitution, and somebody who was also deeply loyal to the president, and just somebody who is a delight to work with.”