As charter schools continue to expand, new research indicates liberal opponents are failing to make effective arguments aimed at curbing the education reform movement.

Opponents' strategy of stressing the role of private companies in running the schools has been largely unsuccessful, said Sarah Reckhow, political scientist at Michigan State University and lead author of the study.

Supporters, meanwhile, have created enduring support for charters partly by pointing out that most charter teachers are not unionized. The number of charter schools in the United States has more than doubled in the past decade, to more than 6,400 today, according to the National Alliance for Public Charter Schools.

Reckhow said the charter school issue is split along ideological lines -- conservatives tend to support it, while liberals oppose it -- which is ultimately bad news for opponents.

"Conservatives outnumber liberals in this country, and only liberals tend to oppose charter schools. They are failing to persuade even fellow Democrats who are more moderate," said Reckhow. "Those who want more regulation of charter schools will have to find more effective ways of persuading people because their base is small and their arguments are falling on deaf ears."

Because there are more Democrats than Republicans in the United States, charter school opponents have a better chance of slowing charter expansion if they can persuade more conservative and moderate Democrats to oppose the movement, added Matt Grossmann, MSU political scientist and co-author on the study.

But that could be difficult, Grossmann said. The study found that even people who oppose charters in their own communities tend to support them in other cities, particularly poor urban areas.

"Middle-class families may support charter schools in poorer districts even if they are skeptical about implementing these programs in their own communities," Grossmann said.

The study, which appears in Policy Studies Journal, involved embedded experiments in a survey of more than 1,000 people.