MADISON, Wis. — A bill that would guarantee Wisconsin's divisive school voucher program doesn't expand without legislative approval looks all but dead after Republican leaders said Friday they don't know if their caucus supports the measure.

The GOP-authored legislation sailed through the Senate this fall but has languished in the Assembly for months. State Department of Public Instruction Superintendent Tony Evers has been pressing for a vote before the legislative session ends in mid-March, but Assembly Speaker Jeff Fitzgerald's spokesman said GOP leaders don't know if they have the votes to pass it.

"The official comment is we don't know if the votes are there in the Assembly to pass it right now. We haven't had a full discussion on it," Fitzgerald spokesman John Jagler said.

School vouchers have been a flashpoint of contention in Wisconsin for years. The program provides parents in eligible school districts with state subsidies to defray their children's private school tuition. Conservatives insist it gives children in underperforming schools an alternative. Opponents see it as a no-confidence vote on public education and complain it drains precious state dollars from public schools.

The program had been limited to students in city of Milwaukee schools. But Republicans included language in the state budget this summer that created new qualifying criteria based on a city's size, poverty levels and per-pupil spending. The new benchmarks enabled districts across Milwaukee County and in the city of Racine to join the program.

No school district outside Milwaukee County or Racine has qualified yet, but a DPI analysis found Green Bay and Abbotsford schools are only one criterion short. Those findings have sparked concerns among voucher opponents that if conditions in districts change, more students from around the state could qualify for subsidies.

"The local taxpayers of these districts, and potentially all school districts, could end up with vouchers without any voice or vote in the process," Evers said in a statement last June.

Rep. Robin Vos, R-Burlington, co-chairman of the Legislature's finance committee, said the budget language was never meant to expand vouchers beyond Milwaukee County and Racine.

Still, Vos and Senate President Mike Ellis, R-Neenah, co-sponsored a bill that would limit voucher participation to school districts in the program when the legislation took effect. Any schools that would want to get into the program after that would need separate legislation. Ellis said the bill would give lawmakers more options, allowing them to extend vouchers for students at a failing school in an otherwise successful district.

The Senate passed the bill in October on a voice vote with no debate. Republican Gov. Scott Walker has said he would sign the legislation into law, but it's gone nowhere.

Ellis drew attention to the measure this week when media outlets reported he was overheard in a Madison hotel telling other lawmakers that Green Bay Preble High School was a "sewer" and could be placed in the voucher program under the bill.

Evers released a statement Friday defending Preble's reputation and imploring the Assembly to vote.

Walker's spokesman didn't immediately return a message seeking comment on the impasse. Phone calls to Ellis' Capitol office and district number rang unanswered Friday afternoon. His cellphone would not accept messages.

Vos spokeswoman Kit Beyer reiterated that Republicans meant to limit voucher expansion in the budget. She said Vos has asked the Legislative Fiscal Bureau to analyze the budget language to see whether additional legislation limiting expansion is necessary.