If school voucher programs were judged by their record, we’d have more of them, not fewer. But politics always seems to get in the way.

For 16 years, Milwaukee has been providing school vouchers for low-income families so that their children can attend private schools. But despite all the data that show how meritorious the Parental Choice Program is, it’s been an uphill battle to expand the program: for all these years, only 15,000 public school students have been eligible for vouchers. Finally, Wisconsin Governor Jim Doyle did the right thing and agreed to a compromise proposal that would increase the number of eligible students by 50 percent, to 22,500.

In two independent studies, Harvard scholars Caroline Hoxby and Rajashri Chakrabarti have been able to demonstrate that the Milwaukee experiment has succeeded in improving the test scores at the public schools most threatened by the program. Competition, which explains the success of the American economy vis-à-vis other nations, also explains why public schools do better when faced with stiff competition from the private sector. And yet the teachers unions resist.

The same phenomenon is being observed in Pennsylvania. Four years ago, the state took over Philadelphia schools, turning the worst 45 of them over to private operators. The effect on the remaining 200 public schools was remarkable: they improved twice as fast as any other schools in the nation’s 10 largest cities. Once again, competition made all schools better.

Despite these successes, school choice programs in New York State are going nowhere. Because of strong opposition to vouchers, proponents of school choice turned to tuition tax credits as a remedy. Governor George Pataki, who has done nothing in all his years in office to help these programs, finally decided to propose a workable tuition tax credit program. But then he ran up against New York Assembly Speaker Sheldon Silver, and everything fell apart. Silver wanted assurances that if some funds were made available, private schools would not raise their tuition.

In short, the road to school vouchers is a political minefield.