THERE WERE a couple of very good reasons for Sen. Sam Brownback (R-Kan.) to set aside his idea of allowing D.C. students to use federal vouchers at private schools outside the city. First, no city official, including the two leading advocates of school vouchers -- Mayor Anthony A. Williams (D) and Board of Education President Peggy Cooper Cafritz -- wanted it to happen at this time. On the grounds of home rule alone, the senator, who is chairman of the Appropriations subcommittee on the District, was well-advised to hold off on acting unilaterally. Second, the federal program is still in its five-year trial period; the first group of nearly 1,000 low-income voucher students enrolled only last fall. Initiating a major change such as what Mr. Brownback proposes might be premature, especially since the existing experiment has not been fully evaluated. The idea of vetting the expansion proposal in public hearings where proponents and opponents can air their views, which Mr. Brownback now calls for, is a better approach.

Meanwhile, supporters of the federal school voucher experiment can take heart in the responses of parents and students thus far. According to officials of the Washington Scholarship Fund, the nonprofit organization that operates the voucher program, as many as 80 students offered scholarships for this fall may be unable to use them because private high schools in the District lack space. The space crunch will only worsen as elementary school students now enrolled in the program move up into higher grades. Concern about the capacity of D.C. private schools to accommodate the demand is, therefore, well-founded. It appears, however, that at this point in the year any congressional action to change the program's rules will not occur in time to assist voucher recipients looking for high schools this fall. For that reason, too, Mr. Brownback was right to hold off.

In the meantime, voucher supporters in the District would do well to determine if private or religious schools in the city can, with extra effort, increase their enrollment capacity. It would be unfortunate if growing parental and student interest in the federal voucher program comes up against an unyielding stone wall erected by schools controlling high school spaces. The rules of the program allow taxpayer grants of up to $7,500 for individual students; Mr. Brownback, as part of his expansion, wants to lift the $7,500 cap. The ramifications of raising federal voucher grant levels and allowing them to be used outside the District are, of course, unknown. That makes it critical that the issue be opened for discussion with city leaders as well as local and regional supporters and opponents of federally supported vouchers, as Mr. Brownback now plans to do.