Massachusetts charter schools are a proven, effective mechanism for bridging the achievement gaps politicians have long talked about.

With President Barack Obama making the lifting of state charter caps a criterion for receipt of $4.35 billion in federal Race to the Top (RTTT) grants, the pressure to allow more of them is on. Still, legislators fear the reaction from school superintendents and teachers unions.

The education establishment would rather lose out on up to $350 million in RTTT funding, even though most of it would go to district schools rather than charters, and even though a brutal recession has led to deep cuts in state programs.

Note to Beacon Hill: This should not be a hard decision. The tired argument that charters drain money from district schools is in tatters. Every time a regular public school loses a student to a public charter school, the state kicks in the equivalent of two years of full funding for the phantom student.

Now Massachusetts Teachers Association (MTA) has issued a report decrying high “attrition” rates in some charter schools. Attrition? You mean students moving around?

Then there’s the fact that the average attrition/mobility rate in Boston’s unionized district schools (22 percent) is higher than in the charters cited by the MTA.

The report further insinuates that high-performing charters push poorly performing students out. The evidence suggests something different. Kids leaving the charter schools criticized by the MTA have, with rare exceptions, already passed the MCAS.

So where do students leaving charters go?

Evidence suggests that only about 10 percent drop out, a number well below the state average and a far cry from Boston’s 35 percent dropout rate.

Either they move to other districts or they return to regular public schools because they want to graduate without having to fulfill the requirements some charters have such as passing a college-level course.

One way to deal with the disruption that may result from kids returning to district schools late in the year is for Boston Superintendent Carol Johnson to work with charter leaders to establish a date certain after which students would be prohibited from switching to a district school to get a diploma.

The MTA should show some contrition about its report on attrition. With 8,000 kids waiting for charter school seats and 12,000 on METCO waiting lists in Boston, the union would be well-served to focus on improving schools, not drumming up weak arguments against the competition.

Their obstructionism is endangering the ability of state schools to access a potential windfall of hundreds of millions of federal dollars. The Race to the Top money would benefit all public school students in the midst of a severe fiscal crisis. If we don’t get the money, we’ll know who to blame.