LITTLETON, Colo. — In this suburban election, lawn signs are being stolen and minivans vandalized. One candidate says she received an email telling her to get cancer and die. Money from the billionaire Koch brothers is funding one side’s commercials and fliers, and upset parents, teachers and labor unions are pouring in cash for the other.

The question facing voters is whether to oust a polarizing school board that has championed charter schools, performance-based teacher pay and other education measures supported by conservatives.

But the vote here in Jefferson County, just west of Denver, has become a money-soaked proxy war between union supporters and conservative groups like the Koch-backed Americans for Prosperity, testing whether parents in an election-year battleground believe a rightward turn in their schools has gone too far.

“It’s a fight to the death,” Tina Gurdikian said one rainy afternoon as she and her daughter who is in the seventh grade, Greta, knocked on doors here in JeffCo, the nickname for Colorado’s second-largest school district, and passed out sample ballots urging voters to recall the board’s three-member conservative majority.