Charter schools have traditionally enjoyed bipartisan support. That's because they deliver results: Students who attend nonprofit charter schools on average learn more and have higher college graduation rates than kids at traditional public schools.

There are signs, however, that this support is in need of shoring up. According to a survey of 4,200 Americans released this month, public support for "the formation of charter schools" has declined by 12 percentage points over the last year, to less than 40 percent. For the first time, more Democrats oppose charters than support them. Even among Republicans, who once hailed charters for introducing competition into the public-school system, support has fallen to less than 50 percent.

Charter schools are, to an extent, victims of their own success. The number of students attending charter schools has doubled in the last decade, to more than 2.5 million. In 14 big cities, including Philadelphia, Washington and Detroit, charters now enroll more than 30 percent of all public-school students.

This has not made them immune to attack. Because charter schools operate independently, their growth poses an existential threat to teachers' unions. The country's largest union has called for a moratorium on the establishment of some new charters, a cause picked up by other advocacy groups. Last fall, opponents of charters helped defeat a ballot referendum in Massachusetts that would have lifted caps on the number of new charters in the state. That President Donald Trump and Betsy DeVos, his education secretary, support charter schools, meanwhile, has soured Democrats on them even more, tarring charters unfairly by association.

Such resistance is myopic — and it's particularly damaging to the poor students, often black and Hispanic, most helped by charters. In urban school districts, African-American students in poverty who attend charters gain two months' worth of learning in math and 44 days in reading, compared with their peers in traditional schools.

Of course, not all charters succeed. One recent study found that students at schools run by for-profit entities — about 20 percent of all charter students — were worse off than if they had stayed in traditional schools. But this is due, in some measure, to the fact that not all states have adopted strong accountability standards.

It's therefore critical that charter supporters make clear their support for laws that hold charters to the same standards as traditional schools and require charter operators to shut down mismanaged and underperforming schools. Funding decisions should be data-driven, so that public resources go to charter-management companies that have a track record of success. By bringing innovation into the public-school system, charter schools can raise the performance of traditional schools as well.

Charter schools were created to improve public schools, not replace them. Parents, students and public officials need to be reassured of this. Democrats ought not to abandon charters simply because Trump supports them; Republicans ought not to settle for lax laws that will undermine education in the end. It's a vital task: The alternative is a less dynamic, less effective public-school system that's even more vulnerable to political assault. In education, reform is a terrible thing to waste.

FROM A BLOOMBERG VIEW EDITORIAL