More than three-quarters of the nation's public schools could soon be labeled "failing" under the federal No Child Left Behind Act, the Obama administration said Wednesday as it increased efforts to revamp the signature education initiative of President George W. Bush.

The projection from Education Secretary Arne Duncan amounted to a declaration that the school-ratings revolution Bush began nearly 10 years ago is itself in jeopardy because the law has become unworkable. President Obama is pushing to loosen accountability rules for most schools but crack down harder on the worst.

The initiative, a major priority for Obama, has been overshadowed by fights over the budget, health care and other issues. By warning that No Child Left Behind might soon require most public schools to be labeled failing, the administration hopes to galvanize lawmakers to act on his plan.

"This law is fundamentally broken, and we need to fix it, and fix it this year," Duncan told the House Committee on Education and the Workforce. "The law has created dozens of ways for schools to fail and very few ways to help them succeed. We should get out of the business of labeling schools as failures and create a new law that is fair and flexible and focused on the schools and students most at risk."

Duncan's estimate that 82 percent of schools could miss academic targets this year, up from 37 percent last year, was based on an Education Department analysis.

No Child Left Behind forces benchmarks for schools to rise steadily nationwide in an effort to ensure that all students receive an adequate education. But critics say the law fosters a blunt pass-fail approach that does not account for academic growth and penalizes schools that are performing well by most measures.

Defenders of the law say Obama's proposals could let too many mediocre schools off the hook for not helping their neediest students.

"If we're going to try, in the name of closing the achievement gap, to whitewash the underperformance of schools, that's really regrettable," said Margaret Spellings, who was education secretary under Bush.

Several analysts and advocates dismissed Duncan's estimate as hype intended to scare Congress into embracing Obama's plan.

"I find it hard to believe," said Jack Jennings, a former Democratic congressional aide who is president of the Center on Education Policy, an independent think tank that tracks the law. "I think they really stretched it for dramatic effect."

Education officials rejected that criticism, saying the analysis was conservative.

Efforts in Congress to rewrite the education law have sputtered in recent years, but there are some signs of momentum from both parties. Sen. Lamar Alexander (R-Tenn.), a former education secretary, has pledged to cooperate with Democrats on the issue. Sen. Tom Harkin (D-Iowa), chairman of the Health, Education, Labor and Pensions Committee, has said he wants to introduce a bipartisan bill before Easter, according to an aide.