Many of the Democratic presidential candidates have been critical of charter schools, and you might wonder why. After all, they are public schools and have a relatively good track record.

David Osborn, writing in the Wall Street Journal, documents some of the charges made against charter schools. Senator Elizabeth Warren argues that these schools “strain the resources of school districts.” Senator Bernie Sanders also says that the growth of charter schools “has drained funding from the public-school system.” This is interesting since the only difference between charter schools and other public schools is the fact that they can operate independently of district bureaucracies.

Of course, that is the real reason. Osborn reminds us that no interest group has more clout in Democratic primaries than teacher unions. “In the last presidential election, the American Federal of Teachers and National Education Association combined spent $64 million.” Charter schools are free to unionize but rarely do. Only about 11 percent of them chose to do so. As the charter school movement grows, union influence shrinks.

Charter schools are quite successful and provide an alternative to millions of students (two-thirds of which are nonwhite). “Graduation rates, college-going rates, and college completion rates are also higher among students who enroll in charters schools.”

There is another benefit: competition. We don’t have true school choice and won’t get it any time soon. But the presence of charter schools does provide some competition to the public school. It encourages school leaders and teachers to improve their schools so they are more attractive to parents wanting to give their children the best education they can receive.

Charters schools don’t deserve the criticism they are receiving these days from many of these candidates.