Students who spent two years in private schools courtesy of the nation’s only federally funded voucher program saw no significant benefits overall, though several particular types of children did benefit, a new study has found.

The findings come as the future of the program — a District of Columbia scholarship fund financed by Congress that now helps nearly 2,000 low-income children attend private schools free of charge — is seemingly in jeopardy, with teachers unions and some congressional officials mounting efforts to block reauthorization when its five years expire next year.

The District of Columbia’s delegate to Congress, Eleanor Holmes Norton, is today outlining a plan for how to end the program in an op-ed published in the Washington Post, an aide said; supporters, meanwhile, including the education secretary, Margaret Spellings, are mobilizing to keep it going.

The study, the latest in a series of reports on the program, provides ammunition to each side.

Most damningly, the study found that after spending two years in a private school, students showed no statistically significant academic gains when compared with their peers who applied to the program but were not selected.

Among the positives was that the program increased relative parent satisfaction by an even higher margin than was found in last year’s study, which looked at just one year’s worth of experience.

The study also found that some groups of children did benefit academically, getting a boost equivalent to between two and four months’ worth of extra schooling.

Researchers said those that benefited appeared to be, in general, more motivated and better prepared for private school than students who did not benefit.

These included students eager and savvy enough to have applied to the program in its first year; students who entered the program with relatively higher academic performance, as judged by a pretest, and students who came into the program from public schools good enough to pass the standards devised by the federal No Child Left Behind law.

The study also offered a glimpse at the kinds of private schools into which voucher students are enrolling. The schools range from relatively low-tuition Catholic schools to some of the district’s most prestigious prep schools, including Sidwell Friends, the alma mater of Chelsea Clinton, and St. Albans, the alma mater of Vice President Gore.

Compared to the public schools students would have attended, the private schools tend to be more racially mixed, smaller, and to have smaller class sizes, according to the study.

Teachers unions and many Democrats oppose vouchers, saying they drain resources from public schools.

Some Democrats are starting to express support for vouchers. One, Kevin Chavous, a former Washington, D.C. city council member, is now the board chairman of the New York-based lobbying group Democrats for Education Reform.

“It’d be a national tragedy if Congress interceded to pull the rug out under these kids,” Mr. Chavous said. “I hope that politics don’t get in the way of our children’s future.”

A mother with two children in the program, Wendy Lewis, said she would move out of her home in D.C. to find better schools.

“I’m very worried,” Ms. Lewis said. “It’s not looking so good, but we are fighting for it.”