A nonpartisan primary election in Arizona could lead to "very modest" change, affecting 10 percent to 20 percent of the contested races in the state, says a political scientist studying the proposal.

"It might encourage people to vote for the more moderate candidate," says David Berman, senior research fellow for the Morrison Institute for Public Policy at Arizona State University.

"In districts where you have two people from the same party, chances are you probably will get the most moderate candidate elected, because you'll have independents and people from the other party making that choice, as well as the regular party members, and they're likely to skew it."

But, he says, "maybe 10 to 20 percent of the time, it will involve giving people a choice between two members of the same political party."

Berman has researched the potential effects of passing Proposition 121, known as the open primary or top-two system, in a paper he wrote for Morrison called Top-Two Proposition: What Nonpartisan Elections Could Mean for Arizona.

If approved, the measure will eliminate primary elections run by political parties. All candidates will run on one ballot, and all voters will decide among them. The top two vote getters will face off in the general election, regardless of party affiliation.

Its proponents say the change is needed to break what they call a logjam of extremism on both ends of the political spectrum that characterizes state politics.

Berman says the change will bring more competition to the electoral process, at least in primary elections.

"It'll make the primary more competitive, certainly," he says. "Rather than people separating out by party, all the candidates will be in one primary. So that you'll have an awful lot of choice. ... You'll get your choice of all the candidates. That will be a lot more competitive."

He says third-party candidates likely will have a difficult time getting through the primary and onto the general election ballot, so there may be less choice there.