A leading Senate Republican is considering changing the D.C. school voucher program by increasing the $7,500 annual limit on scholarships and allowing students to use them at private schools outside the city.

Sen. Sam Brownback (R-Kan.), chairman of the Appropriations subcommittee on the District, said expansion is needed because a shortage of high school slots could leave as many as 90 students who have been awarded taxpayer-funded grants with no school to attend this fall.

Two key Democratic senators on the panel said they would oppose such measures, arguing that Congress should wait until the five-year pilot program has run its course before considering changes.

In an interview, D.C. Mayor Anthony A. Williams (D) also expressed concern about any alterations, but his spokesman later said that Williams has not ruled out such options.

The voucher legislation passed by Congress last year provides low-income D.C. children with federal grants of up to $7,500 to cover tuition and other expenses at private schools in the city. Nearly 1,000 students and 53 schools participated last year, and those figures are expected to climb to 1,600 students and at least 67 schools this fall.

According to the Washington Scholarship Fund, the nonprofit group that administers the vouchers, high school tuition is generally much more than $7,500, and a higher voucher limit could help students who cannot find a high school in the District to accommodate them.

Sally Sachar, president and chief executive of the fund, said the proposal would not increase the $12.1 million annual federal appropriation for the vouchers because the scholarship fund would use $5 million left over from the first year of the program.

"What we want to avoid is having families wanting to use a scholarship and not having a program to choose," Sachar said. "We've been exploring with the mayor and people on the Hill what options might be available to solve this problem."

Brownback is reviewing changes that could be made to the $9 billion D.C. budget bill once Congress returns from its Independence Day recess next week.

For instance, he said, 15 private high schools could be added by expanding the program three miles outside D.C. boundaries into Northern Virginia and suburban Maryland.

"If you're going to see how the program really works, you've got to have slots for children to enroll in," Brownback said. "We could have up to 90 kids with vouchers and no high school to go to, and that's not fair to test the program that way."

U.S. Education Department spokeswoman Susan Aspey said senators have "talked to us about some ideas to address capacity issues in the program."

A Senate battle over vouchers could put an obstacle in the unusually clear road the D.C. budget bill has had this year. Last night, the House neared final passage of the District's 2006 budget, a spending plan that includes $8.3 billion in local funds and $560.3 million in federal aid, which is part of a larger $60 billion spending bill funding the transportation, treasury and housing agencies, among others.

In 2003, approval of the D.C. budget was held up for months while the White House, a Republican Congress and Williams pushed through passage of the voucher program by a one-vote margin in the House.

On Tuesday, the mayor said he had strong reservations about possible Senate changes to the program.

"Why don't we leave well enough alone?" Williams said. "We talked about testing this out over a period of three [more] years. I mean, Jesus, Lord -- that's my initial reaction."

Yesterday, however, spokesman Vince Morris called to say that Williams "doesn't like it, but he's not yet opposed to it. . . . He's looking at all options that will ensure that kids who want to pursue their education have the opportunity and are not frozen out."

Other Democrats were less supportive of changes in the program.

Sen. Mary Landrieu (La.), the ranking Democrat on the D.C. subcommittee, said the proposals "raise important questions. . . . However, we are only in the second year of a five-year demonstration period, and changing the parameters of an experiment in midcourse could potentially invalidate our findings."

Sen. Dianne Feinstein (D-Calif.), a key supporter of the voucher legislation that received final congressional passage last year, said: "I am opposed to any expansion of this program. It is a one-time, five-year pilot project, and the results should be duly considered prior to any request for expansion."

Sachar said the scholarship fund is deferring to elected leaders to decide the best way "to ensure that the program works from the perspective of schools and families." But, she added, "there may be a mismatch in supply and demand between students looking for spaces and the spaces available. . . . We may not have enough capacity in the District."