WASHINGTON — Congressional Republicans this week proposed a $100 million plan to let poor children leave struggling schools and attend private schools at public expense.

The voucher idea is one in a series of social conservative issues meant to energize the Republican base as midterm elections approach.

In announcing their bills, House and Senate sponsors acknowledged that Congress likely won't even vote on the legislation this year.

Still, the move signals a significant education fight to come. GOP lawmakers plan to try to work their voucher plan into the No Child Left Behind law when it is updated in 2007.

"Momentum is on our side," said Rep. Howard McKeon, R-Calif., the chairman of the House Education Committee.

The Bush administration requested the school-choice plan, but the media event held Tuesday caused some awkwardness for the Education Department. The agency just released a study that raises questions about whether private schools offer any advantage over public ones.

Under the new legislation, the vouchers would mainly go to students in poor schools that have failed to meet their progress goals for at least five straight years.

Parents could get $4,000 per year to put toward private-school tuition or a public school outside their local district. They also could seek up to $3,000 per year for extra tutoring.

Supporters say poor parents deserve choices, like rich families have. When schools don't work, said Education Secretary Margaret Spellings, "parents must have other opportunities."

During Bush's presidency, Congress approved the first federal voucher program in the District of Columbia, and private-school aid for students displaced by Hurricane Katrina.

So far, Congress has refused to approve Bush's national voucher proposals. The new one is the first to target money for kids in schools that have fallen short under federal law.

Critics dismissed it as a gimmick.

"Voucher programs rob public-school students of scarce resources," said Reg Weaver, president of the National Education Association, a teachers union. "No matter what politicians call them, vouchers threaten the basic right of every child to attend a quality public school."

Meanwhile, Spellings faced questions about her department's handling of a new study comparing students in public and private schools that had been quietly released on Friday.

The study found that, overall, private-school students outperform public-school children in reading and math. But public-school students often did as well, if not better, when compared to private-school peers with similar backgrounds.

The study had many caveats and warned that its own comparisons had "modest utility."