In this 2015 file photo, fifth- and sixth-grade students warm up for class at For Academy for Ideal Education, a private school in D.C. (Astrid Riecken/For The Washington Post)

Congress is expected to extend the D.C. school voucher program as part of a bipartisan budget deal this week, a move that follows the release of a new federal analysis showing that some voucher recipients in private schools trailed their public-school counterparts on standardized tests.

The legislation would re­authorize the D.C. Opportunity Scholarship Program, which helps 1,100 low-income students attend private schools, through fiscal 2019. The program is the only federally funded effort of its kind.

The congressional deal would also continue funding at current levels through September, sending a total of $45 million to the District for education, split among D.C. Public Schools, D.C. public charter schools and the voucher program. The program gives poor children up to $8,452 to attend a private elementary or middle school and up to $12,679 for high school.

Advocates for the voucher program said they were relieved after spending years fighting for its survival under President Barack Obama, who opposed vouchers. President Trump and Education Secretary Betsy DeVos, in contrast, have pledged to spend billions of dollars expanding vouchers throughout the country.

“It’s hard to get schools to sign up for the program and hard to get parents to sign up their kids, if there’s uncertainty,” said Kevin Chavous, who lobbied Congress to create the voucher program in 2004, when he was a D.C. Council member.

But critics said the reauthorization was galling given the results of the new study by the Education Department, which suggested that the voucher program had a negative impact on student achievement after one year, especially in math. The D.C. study, released last week, came on the heels of similar findings on voucher programs in Louisiana, Indiana and Ohio.

“Not only do students not do any better in the voucher programs, but it’s actually been proved that they’re doing worse,” said David Grosso (I-At Large), chairman of the council’s education committee and an opponent of the program. “I am always disappointed when Congress intervenes in D.C. local matters, and I think this is one of those areas where it is extremely consequential.”

Mayor Muriel E. Bowser (D), a supporter of the program, said, “Families are choosing schools, and they know what they are choosing.”

Chavous and other voucher advocates have said that the study’s findings cannot be seen as a verdict on the program’s effectiveness, because they are a snapshot of only one year. They also say math and reading test scores are a limited measure of school quality.

The D.C. study was conducted using what’s known as the gold standard in scientific research: An experimental design, comparing the performance of students who received a voucher through a citywide lottery to the performance of their peers who applied for a voucher and didn’t receive one. The study was designed to comply with the law as currently written, which requires the “strongest possible research design” for determining the vouchers’ effectiveness.

The reauthorization rolls back that language and prohibits the department’s researchers from using that gold standard. Instead, it says that researchers must use a “quasi-experimental” design, comparing voucher recipients to students with “similar backgrounds” in D.C. public and public charter schools.

Researchers say this approach is generally weaker because it creates uncertainty about whether the comparison is fair. “This program has been studied rigorously since it began in 2004, using an approach that the field of medical research would regard as common practice,” said Mark Dynarski, who co-authored the D.C. study released last week. “If rigor is rolled back, a future study might lead to more questions than answers.”

Rep. Gerald E. Connolly (D-Va.) called the research change an “egregious dilution” of serious science, accusing his Republican counterparts of trying to escape empirical data that might not back up their school-choice philosophy.

But voucher advocates said that they had good reason to seek changes in the study of the program. In the past, they said, children who lost the voucher lottery were then unable to get a voucher in subsequent years, because that would interrupt the research. And they had to keep coming back each year to take a standardized test.

That was a lot to ask of children who had not benefited from the vouchers, said Rachel Sotsky, executive director of Serving Our Children, the D.C. nonprofit organization that administers the money. “There are a lot of problems with sticking with the gold standard,” she said.

The reauthorization proposal would require voucher recipients to take the same standardized test that is administered in D.C. Public Schools, which is currently a Common Core-aligned exam known as PARCC. The provision would ease apple-to-apple comparisons but may be unpopular with private-school administrators, some of whom may feel they are at a disadvantage because they don’t teach to the test.

The proposal also would add more measures of accountability for schools in which more than 85 percent of students are paying with vouchers. Serving Our Children would be required to develop a way to scrutinize the financial viability of those schools, and evaluators would be asked to examine the academic achievement of students at that subset of schools.

Alejandra Matos contributed to this report.