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.

In the House, the new Republican majority is still canvassing the school-reform views of a freshman class that is highly skeptical of the government’s role in education. That could pose one of the largest obstacles to passing a bill.

But House Speaker John A. Boehner (R-Ohio), who helped write No Child Left Behind, has largely refrained from criticizing Obama’s reform proposals. On Wednesday, Rep. John Kline (R-Minn.), the education panel’s chairman, praised Duncan’s efforts. “Although we may not always see eye to eye,” Kline said in the hearing, “you and I share a belief that the current system is broken and in desperate need of repair.”

Obama’s plan calls for schools to be rated on how much academic growth their students achieve. Those that excel would be rewarded, the vast majority in the middle would be given more flexibility to choose strategies to improve, and the lowest performers would face a stricter federal mandate to adopt a stringent school turnaround program.

The No Child Left Behind brand has deteriorated since Bush left office in 2009, but many educators agree that the law’s focus on standardized testing and minority achievement gaps shined a critical spotlight on problems that public schools have long sought to avoid.

There is also widespread agreement that the typical school will never attain the ideal enshrined in the legislation — that all students should become proficient in math and reading.

Under the law, schools must test students in reading and math in grades three through eight and once in high school. Those scores, plus attendance and graduation rates, are used to determine whether a school is making what the government calls “adequate yearly progress” toward a goal of 100 percent proficiency by 2014.

The legislation requires schools to show that various groups of students — including the poor, racial minorities and those with disabilities — are on track. Schools that miss the targets face a series of sanctions, the most severe of which are a state takeover or a shutdown. The law, however, grants states much leeway to meet targets, including a method known as “safe harbor,” which rewards schools for making a minimum degree of progress.

The Center on Education Policy reported that 28 percent of schools nationwide missed the
No Child benchmarks in 2007, 35 percent in 2008 and 33 percent in 2009. The center estimates that at least 37 percent fell short last year. In the Washington area, the center found that the failure rate in 2009 was 23 percent for Maryland, 28 percent for Virginia and 75 percent for the District.

Charles Barone, a former congressional aide who helped draft the 2002 law, called Duncan’s projection “fiction.” Barone tracks federal policy for a group called Democrats for Education Reform, which is generally in accord with Obama’s policies on education changes.

“He’s creating a bogeyman that doesn’t exist,” Barone said of Duncan. “Our fear is that they are taking it to a new level of actually manufacturing a new statistic — a ‘Chicken Little’ statistic that is not true — just to get a law passed. It severely threatens their credibility.”