With cash pouring into the 2016 presidential campaigns, next year's election is quickly becoming the most expensive in history. But while the focus has largely been on the national candidates and the size of their war chests, special interest groups have spent millions of dollars on campaigns for state offices, ballot initiatives, and other local elections.

In the Denver suburb of Jefferson County, special interest groups have funneled $1 million into a contentious school board race that has turned into a battle over national education reform and the role of government. Much of the money is coming from Americans for Prosperity, the national political advocacy group funded by the billionaire Koch brothers, who are backing three conservative school board members.

The three Koch-backed board members, elected in 2013, are facing a recall effort from parents and activists in Jefferson County who say the officials have attempted to privatize schools, implement a conservative education curriculum, and tie teacher pay to student performance instead of seniority. The president of a Denver think tank called it a "proxy war" between libertarian ideology and education reform.

"I think a lot of people are looking at this as a way to test the waters for what might take place in other areas," school board candidate Susan Harmon told the Washington Post. "To me, this is an in­cred­ibly important election about moving public education in the right direction, not toward privatization."

Most state and local elections fail to attract national attention. But in states like Virginia, where democrats are battling to retake control of the state senate, the outcome of a single district will have important consequences for the 2016 presidential race.

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The balance of power in the upper house of Virginia's legislature comes down to a single race in the state's 7th district. Each candidate has spent more than a million dollars already, making the contest the most expensive in state history, according to local media. A win for democrats would offer the party's presidential candidate a significant leg up in the swing state next year.

Residents of Ohio, meanwhile, are voting on a ballot initiative that could influence the future of the legal marijuana industry. Issue 3 on the state's ballot would legalize weed for both recreational and medicinal users, but it would only allow pot cultivation on 10 farms. The wealthy investors who stand to profit — including ex-boy band star Nick Lachey, NBA legend Oscar Robertson, and former pro football player Frostee Rucker — have spent $25 million pushing for the ballot initiative.

In Houston, residents are voting on a ballot measure that would grant minorities, including transgender people, protections from discrimination in housing, employment, and other areas. The initiative, known as the Houston Equal Rights Ordinance, or HERO, has sparked a national debate that attracted the attention of many high-profile figures, including Democratic presidential candidates Hillary Clinton and Bernie Sanders. Actresses Eva Longoria and Sally Field have also weighed in, as have Apple, Hewlett-Packard, and other major companies.

HERO's opponents have launched campaign ads urging people to vote against it, claiming it would allow sexual predators to use women's bathrooms. Supporters, on the other hand, have raised more than $3 million for their campaign.

While money is influencing many of the local races, the issue of getting money out of politics is the focus of two separate measures in Maine and Washington state. Maine's Question 1, also known as the Clean Election Act, would require political advertisements to list their donors in the ad, while also increasing the amount of public dollars available to fund campaigns. Seattle's Initiative 22 would give each city resident a $100 coupon — dubbed a "Democracy Voucher" — to hand over to the candidate of their choice.

Follow Olivia Becker on Twitter: @Obecker928