But changes were sweeping through the halls of public education before Ms. Rhee took over the leadership of the Washington schools three years ago. So her departure seemed unlikely to slow that momentum, experts said.

“This movement has become so much bigger than one person,” said Timothy Daly, president of the New Teacher Project, a nonprofit group Ms. Rhee founded in 1998 to help school districts hire more effective teachers.

Ms. Rhee was a polarizing chancellor, especially after she fired hundreds of teachers whom she called ineffective. The teachers’ union campaigned against the man who hired her in 2007, Mayor Adrian Fenty, and he lost last month’s Democratic primary.

Ms. Rhee called Mr. Fenty’s defeat a devastating blow to the children of Washington, and hinted that she might resign. She made that official on Wednesday in a news conference, standing with Mr. Fenty and the Democratic mayoral nominee, Vincent Gray, who is now the city’s presumptive mayor.

“We have agreed that the best way to keep the reforms going is for this reformer to step aside,” Ms. Rhee said.

Mr. Gray announced that Kaya Henderson, the deputy chancellor who is a close associate of Ms. Rhee’s, would take over as interim chancellor, and he pledged that school improvement policies Ms. Rhee enacted would continue after he takes office.

In an interview late last month, Ms. Rhee said it had been disheartening to receive e-mails from friends and read media accounts that interpreted Mr. Fenty’s defeat and her likely departure as a major setback for their movement.

“People are calling it a huge blow,” Ms. Rhee said. “But it would be such a tragedy if what people took away from my three years here was that you shouldn’t go too fast. That’s the wrong message. It’s not a time to retreat.”

Ms. Rhee carried out her duties as chancellor with a missionary zeal she said she owed to her conviction that Washington students were being shortchanged.

“You’d wake up every morning and know that kids are getting a really crappy education right now,” she says of her experiences at the Washington schools in the movie “Waiting for Superman.”

During her tenure, she closed dozens of failing schools, negotiated a teachers’ union contract that Education Secretary Arne Duncan praised as innovative, and pioneered a new teacher evaluation system.

“Michelle’s very prominence could now bring a deflating effect,” said Frederick M. Hess, a director at the American Enterprise Institute, a conservative research group. “Because Washington is so central to media markets, enormous attention was paid to all that she did, and the significance of her reforms was massively inflated nationally. Now I’m afraid we’ll see the same process in reverse: if there are reversals of her efforts, that too will attract attention.”

But there are other forceful agents for school change.

The broad changes under way in cities and schools nationwide have been pushed along by other activist superintendents, including Joel I. Klein in New York.

In addition, education philanthropists like Bill Gates and Eli Broad are pouring hundreds of millions of dollars into dozens of districts, coast to coast, to help transform public education.

In some cases, teachers’ unions have been cooperating enthusiastically with school improvement efforts. They have acted as partners, for example, with the Gates Foundation in a half-dozen districts in an effort to develop more effective teacher evaluation tools.

Secretary Duncan has praised the unions for helping to forge innovative features in contracts signed in New Haven, Denver and other cities.

A fast-growing charter school movement now starts hundreds of new schools in neighborhoods and towns every year, competing with traditional schools for students, with the support of the Obama administration.

The Race to the Top education grant competition helped persuade nearly 40 states to adopt common academic standards for the first time, and persuaded a string of legislatures to lift caps on the number of charter schools.

One example puts Ms. Rhee’s contributions into perspective. Washington’s new teacher evaluation system, which combines calculations of student academic growth with classroom visits by teams of evaluators, is now in place across the Washington schools, which educate less than 50,000 students.

During the Race to the Top competition, by comparison, state legislatures passed laws revamping teacher evaluation systems in similar ways across a dozen states, encompassing thousands of school districts that collectively educate perhaps 13 million students.

“I am not a big believer that one individual is somehow the driver” of school reform, Mr. Klein said. “There is a sort of move forward, push back, move forward dynamic here. But the arc is clear to me. National policy, and changes all over the country, are changing what we do with schools.”

A Colorado state senator, Mike Johnston, who like Ms. Rhee is also a Teach for America alumnus, said her work had inspired hundreds of young people to work for education change.

“Maybe Michelle’s greatest contribution is that she is no longer an anomaly,” Mr. Johnston said.