BOSTON — Supporters of a ballot question aimed at expanding the number of charter schools in Massachusetts have spent nearly twice as much on television ads as opponents.

As of Monday, Great Schools Massachusetts, which backs the question, had spent nearly $12 million on broadcast television ads. That’s compared to about $6.5 million spent by the Campaign to Save Our Public Schools, which opposes Question 2.

The groups have spent a combined $18.4 million on television ads, according to data from the Center for Public Integrity.

The ballot question would let Massachusetts add up to a dozen new or expanded charter schools each year outside of existing state caps.

The only other money spent on ballot question ads came from supporters of Question 4, which would legalize the recreational use of marijuana in the state.

Question 4 backers have spent just over $1 million on television ads.

Even though they spent nearly twice as much as opponents, the ads from Great Schools Massachusetts have actually run fewer times — about 2,800 times — than ads from the Campaign to Save Our Public Schools, which have run nearly 3,000 times.

While both groups are advertising in the Boston, Providence, Rhode Island, and Springfield, Massachusetts, markets, most of the money was spent on the Boston ads for both groups.

One reason why Great Schools Massachusetts has spent more to have their ads run less often appears to be that the group is focusing more on the big networks instead of the smaller channels.

Another factor is the time of day, with Great Schools Massachusetts running more ads in prime time.

Great Schools Massachusetts spokeswoman Eileen O’Connor said the ads are part of a larger strategy to educate voters.

“We’re doing everything we can to make sure voters know what Question 2 is all about: giving thousands of parents stuck in underperforming school districts the opportunity to choose a better public school,” O’Connor said in a statement.

Steve Crawford of the Campaign to Save Our Public Schools said opponents know they will be outspent, including by money from out-of-state donors.

“Our focus is on one-on-one conversations with voters concerned about the harm Question 2 would do to our public schools, not the size of our TV buy,” he said in a statement.

The ballot question has divided top elected officials in Massachusetts.

Republican Gov. Charlie Baker is a longtime supporter of charter schools and an enthusiastic supporter of the ballot question while some prominent Democrats, including U.S. Sen. Elizabeth Warren, oppose it.

The difference in television ad buys reflects the difference in overall fundraising totals.

According to reports filed with the state Office of Campaign and Political Finance, supporters of the question have so far raised about $15.5 million while opponents have raised about $7.2 million.

About half of the money raised to support the charter school question comes from the New York-based Families for Excellent Schools. It’s unclear from campaign finance reports where the group’s funding comes from.

Other funders include former New York Mayor Michael Bloomberg and two Arizona-based heirs to the Wal-Mart fortune.

The three biggest donors to the Campaign to Save Our Public Schools are teachers unions: the Massachusetts Teachers Association, the American Federation of Teachers and the National Education Association.

The figures analyzed by the Center for Public Integrity cover ads aired starting on Jan. 1, 2015, but represent only part of the money spent on political advertising. They do not include ads for radio, online, direct mail or TV ads that aired on local cable systems. The estimates also do not include the cost of making the ads.

Election Day is Nov. 8.