Read all of our coverage on 2016 ballot measures at governing.com/ballotmeasures.

Around the country, an increasing number of voters are registering as independents. Should they still have a say in who party nominees should be?

That question was up for debate this fall in Colorado. A pair of measures passed on Tuesday will alter the state's primary elections, allowing unaffiliated voters -- who now outnumber Democrats and Republicans in the state -- to participate.

One measure will require the parties to hold presidential primaries, rather than caucuses. It passed with nearly two-thirds of the vote. The other measure applies to all partisan primary elections, opening them up to unaffiated voters. According to incomplete returns, it has passed narrowly.

"If taxpaying citizens are going to pay for the cost of elections, they should have a say in them as well," said Jessie Koerner, spokeswoman for Let Colorado Vote, the group that backed the measures.

With just over half the states keeping their primaries closed to only those registered with the relevant party, Colorado may not be the last state to revisit this issue.

Both major state parties in Colorado opposed the measures, arguing that the business of nominating candidates is central to the mission of a party and shouldn't be interfered with by people who are not affiliated with it.

"It's like saying the Mormon Church will get to choose who is going to be the Pope or allowing [New England Patriots coach] Bill Belichick to decide who's going to be the quarterback for the Denver Broncos," argued Ted Harvey, a former GOP state senator, during a public radio debate in September.

The U.S. Supreme Court has ruled that parties have First Amendment rights of association that allow only their members to choose their officers and nominees. In 1999, the court struck down a California measure that allowed primary voters to choose candidates regardless of party. That measure, wrote Justice Antonin Scalia, "forces political parties to ... have their nominees, and hence their positions, determined by those who, at best, have refused to affiliate with the party, and, at worst, have expressly affiliated with a rival."

In Colorado, the new laws mean that unaffiliated voters can soon participate in either Democratic or Republican primaries -- but not both. (Parties, however, can close off their nomination process by holding assemblies or conventions instead of primaries. But the measure would require a three-quarters vote of a party's state central committee to approve such a move.)

Among the 20 states that already have open presidential primaries, a few only let independents vote in Democratic primaries. In contrast to presidential primaries, GOP congressional primaries are open in several states, while Democratic congressional primaries are limited to that party's registered voters.

Democratic Gov. John Hickenlooper supported the measures, saying they will lead to increased participation in the political process. Better decisions are made when the maximum number of people participate in democracy, said Kelly Brough, president of the Denver Metro Chamber of Commerce, which backs the measures.

"We have seen how extremes, whether on the right or left, have kept us from making progress," said Brough. "If you could engage unaffiliated voters, you could reward politicians who compromise and find solutions on these issues, instead of people being penalized for working across the aisle."

Forcing politicians to moderate their stances by having to appeal to a broader pool of voters has been the motivation behind electoral changes in other states. In California and Washington state, for example, voters approved ballot measures that created top-two primary systems. That means that, regardless of their party affiliation, the first and second place candidates face off in the general election. As a result, California's U.S. Senate race this year features two Democrats, while Washington's contest for state treasurer has come down to two Republicans.

But academic research suggests that such changes have done little to moderate the behavior of politicians in office, said Kyle Saunders, a political scientist at Colorado State University. There are other incentives in the system that keep politics polarized, such as free-spending outside groups that tend to be highly ideological.

It may sound counterintuitive, Saunders suggested, but strengthening the role of parties could actually help moderate politics. State parties tend to be less polarizing than groups such as super PACs because they have to worry about long-term results and the reputation of the party's brand for more than a single election cycle.

Saunders conceded that the argument in favor of the propositions -- that they would empower more people -- was "very appealing." Especially since Colorado has more than 1 million unaffiliated voters.

Let Colorado Vote raised more than $4 million in support of the measures, including $1.4 million from health-care executive Kent Thiry. The main opposition group, the Citizens for Integrity Issue Committee, raised about $70,000, with more than half its funds coming from the Service Employees International Union.

Read all of our coverage on 2016 ballot measures at governing.com/ballotmeasures.