WASHINGTON -- It wasn't surprising when children entering Head Start in 2000 were among the lowest-scoring 25% of youngsters nationally on vocabulary, early-writing and early-math skills. The federal program is for low-income preschoolers, who often come from unsettled households.

But when school ended nine months later, the Head Start youngsters still were in the lowest quarter nationally -- "halfway to average," Brookings Institution social-policy expert Ron Haskins said.

Those results, released by the Department of Health and Human Services this summer after its most-recent review of Head Start's academic performance, are certain to fuel the debate as Congress grapples with reauthorizing the hugely popular program this fall. Millions of schoolchildren head back to classes Tuesday, putting education back at the center of public debate -- and back in the laps of lawmakers. Congress will take up college-tuition and special-education bills this term, along with a spending bill for elementary and secondary schooling. The debate on Head Start may be the fiercest.

Head Start is an education, health and nutrition program that enrolls about 900,000 youngsters at a yearly cost to taxpayers of about $7,000 each. As in the Medicare overhaul debate, President Bush sees in Head Start a chance to remake a prominent Lyndon Johnson Great Society program in his more conservative image -- in this case, taking power away from the Washington-funded bureaucrats and giving states more discretion to run Head Start and blend it with their own preschool initiatives.

Many nonpartisan and liberal experts, frustrated with Head Start's weak academic showing, think that is worth at least a trial. The proposal has run into a buzz saw of opposition from Head Start operators fearing a loss of control, and from Democrats leery of ceding an education issue to the president.

The complaints have become so fierce that even many Republican legislators "think it's not worth the fight" to promote Mr. Bush's overhaul, said Mr. Haskins, a former Republican Congressional aide on welfare policy. "Head Start is a sacred cow."

The House of Representatives scaled back the president's plan to an eight-state pilot project and passed it by a one-vote margin during July. The Senate is scheduled to take up Head Start this month, but Education Committee Chairman Judd Gregg, a New Hampshire Republican, has said he won't include even a pilot project.

As it is now, the federal government bypasses the states to directly fund 1,570 community groups, school districts, tribes, churches and local governments that operate 19,000 Head Start preschools. The White House plan would let the states run Head Start, much as they now run their elementary and secondary schools, but with the federal government putting up the money -- about $6.8 billion nationally next year, just slightly higher than this year's level.

When Mr. Johnson started Head Start, few youngsters went to preschool. Now, Bush officials reason, 39 states have started their own preschool programs for low-income youngsters apart from Head Start. Many of those programs require better-trained teachers and smaller classes than Head Start does. The states are under pressure from the president's No Child Left Behind law to improve student reading and math scores and to close the huge learning gap between whites and minorities. Giving the states control of Head Start would let them dovetail their preschool and elementary-school programs, the White House says.

Many independent educators agree. "It's silly to run parallel programs aimed at the same universe of children," said Amy Wilkins, head of the Trust for Early Education, a nonpartisan, foundation-funded advocacy group. She hardly is a likely apologist for Bush proposals. Ms. Wilkins used to work at the Democratic National Committee and blasts the president for underfunding Head Start, even as she praises his overhauls.

But the Bush overhaul plan brought a furious response from Head Start operators, who launched a "Save Head Start" Web site, staged fever-pitch rallies and issued flurries of news releases charging that the plan would threaten the futures of needy children. They now are launching a national TV campaign accusing Mr. Bush of planning to "dismantle" Head Start, leaving its students "further and further behind." The changes "would be a disaster," said Ron Herndon, board chairman of the National Head Start Association, which represents Head Start operators and parents.

He said states would be unlikely to improve on Head Start which, in addition to academics, offers health services such as vaccinations and dental care that most state child-care programs don't provide. He worries states could end up gutting the program, if budget shortfalls tempt governors to shift Head Start money to their own programs, meaning fewer children would be enrolled. In fact, 23 states have cut back or slowed the growth of their preschool programs during the past two years because of budget woes, according to the National Conference of State Legislatures.

Politics also is buffeting the debate regarding who is in charge, just as it did 40 years ago. Mr. Johnson wanted local groups, rather than the states, to run Head Start in order to bypass segregationist governors, said Jack Jennings, a longtime Democratic Capitol Hill staffer and now head of the Center on Education Policy, a Washington research group. That system has during the years given Head Start operators wide latitude in how they run their programs and, because they dispense services and provide jobs, political clout that they stand to lose if the states assume control.

With Washington already suiting up for the 2004 election, Democrats fear handing the president any legislative victory, but particularly one on an education bill. Three years ago, Mr. Bush ran on a promise to improve the public schools, co-opting an issue where Democrats long held a polling advantage among voters.

There is a consensus that Head Start needs to strengthen its educational offerings. When Congress last debated Head Start five years ago, there was bipartisan agreement to boost the education requirements for teachers and order them for the first time to teach reading and math skills. This year, Congress and the president want to raise those requirements further.

The House bill would require half of all Head Start teachers to have a college degree by 2008 (about half have only a high school diploma or training-school certificate now). Mr. Bush wants more reading and math in the curriculum. All sides agree Head Start youngsters should be tested at the beginning and end of every school year to measure their progress -- and how good their schools are at teaching them -- although they disagree on what to test.