By The Associated Press

Prominent Republicans are preparing a campaign to oppose an initiative measure that would transform Arizona's primary election, and they'll start by suing to try to keep the proposal off the November ballot.

Maricopa County Attorney Bill Montgomery says the legal challenge to be filed later this month will contend that the initiative unconstitutionally deals with multiple subjects in a single constitutional amendment.

The state constitution says amendment proposals, which must be presented to the voters, may not address more than a single subject. In this instance, Montgomery says, the proposal deals with voter registration, the primary election, the general election and other issues.

Montgomery tells the Arizona Capitol Times that he and ex-congressman John Shadegg will play key roles in the opposition group.

The measure essentially would scrap Arizona's current system of having the primary pick party nominees for the general election.

It would allow voters to vote for any primary election candidate. Only the top two finishers would advance to the general election regardless of party affiliation.