BOSTON – The words “charter schools” won’t appear on any of the ballots cast in Boston’s preliminary mayoral election, but most residents understand the outcome of Tuesday’s vote will have a significant impact on the future of alternative public schools in the city.

It comes down to which two of the 12 candidates voters choose as finalists for November’s general election.

If both finalists are pro-charter, it will give parents and education reformers hope that the city’s charter school cap may eventually be lifted.

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State law currently allows only 18 percent of Boston’s total student population to attend a charter school. The city recently ran out of charter spots, leaving 16,000 students to languish on a waiting list.

A pro-charter mayor could help change that.

However, if the two teacher-union endorsed candidates emerge as the frontrunners, it will signal that Beantown has likely maxed out its charter school opportunities for the foreseeable future.

And if the finalists are on opposite sides of the charter issue? Then voters can expect a bruising battle between reformers and teacher unions, leading up to the final election on Nov. 5.

On the surface, the mayor’s race seems irrelevant to the charter debate. The responsibility for deciding how many of the alternative public schools are allowed to open – and where – falls to the state’s legislators, not its mayors.

But Massachusetts Charter Public School Association spokesman Dominic Slowey tells EAGnews a mayor’s views on charters carries a lot of influence with legislators.

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“If the next mayor of Boston is a charter supporter and supports lifting the cap, it makes it easier to get a bill through the legislature,” Slowey tells EAGnews. “If an anti-charter mayor got elected in Boston, the legislature could defer to the mayor and not lift the cap.”

Bostonians seem to understand that. Polls show education and crime are the voters’ top two issues in tomorrow’s election.

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Parents’ and reformers’ best hope may be city council member and former charter school teacher John Connolly. Not only is Connolly outspoken in his support of expanding the availability of charters, but he also favors other reforms, such as giving school principals greater decision-making power over what happens in their buildings, notes WBUR.org.

Connolly has also shown a willingness to stand up to the Boston Teachers Union. He was the only city council member to vote against the BTU’s most recent labor contract, on the grounds that it didn’t extend students’ learning day. (Longer school days are a hallmark of Boston’s 21, highly regarded charter schools.)

A new poll finds Connolly is the frontrunner in the race, though “as many as nine candidates have a plausible shot at the final,” the Boston Globe reports.

Naturally, the BTU-endorsed candidates – City Councilmembers Felix Arroyo and Rob Consalvo – oppose lifting the charter cap and want to prop up struggling, government-run schools.

That appeals to teacher union leaders, who dislike charters because their teaching staffs are typically not unionized.

The 5,000 BTU members who are eligible to vote in tomorrow’s contest could play a decisive role in determining the winners. If the unionists vote as a bloc, they could account for a large percentage of the 20,000 to 25,000 votes the two successful candidates need, according to political analysts.

Clearly, the teachers union would like to keep Boston’s charter school system from growing. The city’s charters serve as a shining, national example of what the alternative schools can do, especially in troubled urban school districts.

A recent report from Stanford University’s Center for Research on Education Outcomes (CREDO) found the typical Boston charter student learns far more over the course of a school year than his or her public school peers do. That extra learning equates to “more than 12 months of additional learning per year in reading and 13 months greater progress in math,” according to CREDO researchers.

“The average growth rate of Boston charter students in math and reading is the largest CREDO has seen in any city or state thus far,” said Research Associate Edward Cremata in a press release. “These results signify that these schools could serve as a model and have an opportunity to transfer knowledge to not only the rest of the state but to the national sector as well.”

That’s as good an explanation as any about why tomorrow’s mayoral primary is so important to so many people.