DENVER — Three conservative school board members elected on promises to reform teacher pay and boost charter schools in suburban Denver were overwhelmingly recalled in a high-priced election that highlighted the continuing importance of teachers’ unions in education politics.

The fight in Jefferson County, Colorado’s second-largest school district, drew hundreds of thousands of dollars in spending from outside groups, all for offices that aren’t paid. Recall supporters included teacher unions, while the three board members were backed by the conservative powerhouse Americans For Prosperity.

By large margins, voters recalled Ken Witt, Julie Williams and John Newkirk, who had cruised into office in 2013 and made up a majority of the five-member board. Voters also picked their replacements Tuesday, as well as two new members. The new board is now comprised of officials who were against the old majority.

The board majority attracted national attention last year when students walked out of class and, the district claimed, teachers staged sick-outs to protest a possible review of a U.S. history curriculum to promote patriotism.

The trio immediately drew the scorn of liberal groups and some parents by implementing reforms they campaigned on, including giving more money to charter schools and tying teacher pay increases to performance rather than seniority.

Witt, the board president, said he was disappointed by the recall but not surprised.

“Sometimes it’s difficult being the tip of the spear enacting change,” he said.

The board majority considered reviewing new Advanced Placement U.S. history curriculum to emphasize patriotism, although they ultimately didn’t do anything. Board members insisted the outcry had more to do with union displeasure over the new teacher pay system.

Parents and educators formed a group called Jeffco United for Action and collected signatures to force the recall election. They cited the fracas over the curriculum and teacher pay. The group also accused the conservative members of meeting privately before scheduled meetings — a charge the three have denied.

Recall supporters said the elections were about accountability.

“Unfortunately a school board is only accountable to the community, so this was our only option because after two years they wouldn’t listen to us,” said Shawna Fritzler.

Recall attempts on school board officials nationally have become a popular political tool in recent years, said Joshua Spivak, who has tracked the issue at the Hugh L. Carey Institute for Government Reform at Wagner College in New York.

Since 2012, there have been just over 300 recall efforts against school board members, although most never get enough signatures to trigger an election, Spivak said. Of the recall campaigns launched prior to those in Jefferson County, 24 board members lost their seats in elections, another 21 resigned during the signature-gathering process, and 13 kept their seats after votes, according to Spivak.

“The recalls are very effective weapon once you get them on the ballot,” he said.

Tuesday’s results were a dramatic reversal from 2013, when three board members were elected by overwhelming margins.

Money spent and disclosed by groups and individuals opposing the recalls totaled $186,399, including a $70,116 ad buy from Americans For Prosperity, according to figures and documents compiled by Colorado Ethics Watch, an open government group.

Individuals and groups supporting the recall, including teacher unions, spent $277,076, according to Ethics Watch.