Gov. Charlie Baker is calling on lawmakers to expand charter schools in Massachusetts, saying they can help close the achievement gap for children in the state’s underperforming school districts.

Baker testified before the Legislature’s Education Committee on Tuesday as it considered a several bills including a proposal from the Republican governor that would allow for up to a dozen new or expanded charters each year outside the existing cap on the schools.

Boston Mayor Marty Walsh thinks such a plan is too aggressive and wants to see it slowed down. Walsh says he fears that too rapid expansion of  Charters would have the paradoxical effect of putting too much pressure on under-performing districts, possibly setting them up for failure.

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Baker said there were 37,000 children currently on waiting lists hoping to get into one of the approximately 80 charter schools that operate in Massachusetts.

The governor indicated a willingness to negotiate with Democratic lawmakers, but added: “The time is already long past to address this issue.”

Charter school opponents say they drain resources away from traditional public schools.

The charter debate is playing out against a backdrop of a citizens’ petition to place the issue on the 2016 ballot. This would most likely set up a battle royal between the charter reform movement and teachers unions, which oppose charter growth