A November ballot question asking to lift the cap on the number of charter schools allowed in Massachusetts is splitting Democrats into two camps: those who support the the autonomous alternatives to traditional schools, and those who side with public teachers unions who say charters suck resources from already impoverished districts.

That debate is flaring up in liberalism's own backyard and dominating the Democratic primary race for a Somerville and Cambridge-based state Senate seat.

After opening remarks at their July 21 debate that showed little daylight between the positions of the Democratic candidates, the incumbent, Somerville Sen. Patricia Jehlen, and Cambridge City Councilor Leland Cheung, the topic shifted to the upcoming ballot measure to lift the charter cap.

Cheung, the challenger, said he will vote for the measure, and that the Legislature, including Jehlen, failed to pass its own charter legislation to stop the ballot question going forward.

"They had a four-person task force, which the senator was on, to avoid having to go to the ballot and make that decision," Cheung said.

Veteran legislator Jehlen is campaigning against the charter expansion. The senator said Question 2 goes too far by "basically" eliminating the cap on the amount of money that can come out of any district to benefit charters.

"They are not driven by parents. They are not driven by teachers. They are driven by companies that want to expand... and the reason the demand for them is there is, I believe, that parents are told the public schools are failing," Jehlen said, adding that many urban schools are under resourced and serve children from all background and educational disadvantages.

Cheung doesn't assume that parents have been deceived by charter and school-choice rhetoric.

"I've talked to parents who, that charter school represents their hope for trying to get their kid into a better situation educationally. And who am I to say that I'm going to take away a choice from a parent? They're in the best position to judge the options that they have and know what's going to be best for their individual kid," Cheung said.

Cheung was first elected to the Cambridge City Council in 2010. He ran unsuccessfully to become the Democratic Party's nominee for lieutenant governor in 2014. After graduation from Stanford, Cheung worked at Cambridge venture capital firm Masthead Venture Partners, "focusing on digital media, mobile, and internet infrastructure," according to his biography listed on the city of Cambridge's website.

Jehlen has served in the Senate seat since 2005 and had represented part of Somerville in the House since 1991. She graduated from Swarthmore College and earned a Master's degree in teaching from Harvard University. She worked as a history teacher and also served on the Somerville School Committee from 1976 until her election to the House in 1991.

The second Middlesex district includes all of Somerville and Medford, western Cambridge and a sliver of Winchester.

Things got testy during an exchange on the roles public, parochial and alternative education play in the charter debate, when Cheung inferred that Jehlen's children did not go to Somerville public schools.

"I know Sen. Jehlen made a different choice than public schools for her children, but it goes back to choice," Cheung said.

In the 1980s, Jehlen helped found the CHOICE program, a progressive alternative elementary school for public school students in Somerville. Prior to CHOICE being available, Jehlen said she sold bingo tickets to pay for parochial schools, but moved her sons to public school once she began running for office. 

After the debate, Jehlen said she didn't think it was fair for Cheung to make the comment about her children's schools.

"He seems to think that I don't think other people should have a choice," Jehlen said.

Cheung was far from a full-throated advocate for raising the charter cap in 2014, when he responded to a candidate questionnaire from The Springfield Republican saying that there is a place for charter schools to be innovative "but we cannot expand them when it comes at the expense of the entire system."

"I believe in public-private partnerships but privatization mechanisms for government services only work when they do not come at the expense of delivering that same service to people through public means," Cheung wrote in 2014.

Jehlen voted in favor of a Senate bill this year that would increase the charter cap alongside funding for public schools. The bill was not taken up by the House, allowing the ballot question on the cap, which does not include any funding mechanism, to proceed.

In his opening remarks, Cheung highlighted income inequality as his chief concern.

"I don't think there's one silver bullet," Cheung said. I think there's a million things we'll need to do, but that is the issue of our times that we'll need to address."

Jehlen emphasized her record of working within the Legislature to accomplish change while also focusing on the gap between rich and poor in Massachusetts. Preschool for all, she said, is the most effective way to eliminate the income gap for generations to come.

The state, Jehlen said, just doesn't have the resources she thinks are required to provide the services people need.

One of the only politically plausible ways the state could become flush with funds to pay for both education and transportation is the so-called "millionaire's tax," on the richest earners in the state.

Jehlen agreed that schools need more funding and said the movement toward bringing a surtax on incomes of over $1 million before voters in 2018 is the way to get there.

"Either the Legislature has to raise taxes or the people have to raise taxes by passing the 'fair share' amendment in 2018," Jehlen told WGBH News in an interview.

Without that infusion of revenue from the state's richest, Jehlen said, Massachusetts lawmakers just won't have the money to pay for education, transportation, home-care and other things that make the state a livable community.

Cheung called on the Legislature to increase funding for traditional public schools and to invest in education to help students and lure more private companies to Massachusetts.

Like Jehlen, Cheung said the way to close the income gap is to invest more in schools, providing both universal prekindergarten and schools funded at 2002 levels, the high point before budget cuts forced education spending down.

"I'm going to ask Leland at some point where he gets a couple of billion dollars extra every year or even $3 billion, which would take us back to where we were in 2002," Jehlen responded. 

"We don't have the money to keep spending the money that we need to make our transportation system adequate," Jehlen said in response to another topic of great interest to Somerville, Cambridge and Medford—the proposed Green Line extension from Lechmere. The project was scaled back this year in order to keep it's skyrocketing budget under control.

Throughout the race, Cheung has been holding the Legislature and Jehlen herself responsible for not preventing the Green Line quagmire.

Jehlen said that while the cost overruns have been frustrating, her job isn't to manage the project but to empower the executive branch to better administer MassDOT and the MBTA.

"I think Leland doesn't understand the constitution," Jehlen said. "I am not an administrator, I am a legislator."

Cheung and Jehlen will square off at the polls Thursday, September 8.

Correction: A previous version of this story incorrectly suggested the Massachusetts charter schools are operated predominantly by for-profit companies. They are not.