Proponents of Proposition 14, such as Republican Gov. Arnold Schwarzenegger and Lt. Gov. Abel Maldonado, argue that the change would benefit moderates who often stumble in primaries where party activists have an oversized say in the outcome.
"When there's an open primary, a top two, you better have three qualities: be open-minded, be reasonable and be pragmatic, because now you have to connect will all the voters," said Maldonado, a former Republican lawmaker and rancher.
Understandably, the political parties oppose the idea, fearing a loss in clout. That's created an odd alliance in which Republicans and Democrats have joined to try to defeat the measure. The parties argue that if their organizations are marginalized, special interests will fill the void.
"Political parties are important for three key reasons -- they are broad-based, democratically governed and transparent," said Ron Nehring, chairman of the state Republican Party. "You cannot say the same of special interest groups."
An anti-incumbent, anti-Washington sentiment has prevailed this election year, with several tea party-backed candidates knocking off those tapped by the political establishment. The California ballot initiative reflects the frustration with partisanship and the political system.
Maldonado is considered a moderate for his occasional willingness to work with Democrats, a quality that makes him vulnerable in GOP primaries. He drew the wrath of fellow Republicans last year when, as a state senator, he agreed to vote for nearly $13 billion in tax increases if lawmakers and the governor put Proposition 14 on the ballot.
Schwarzenegger, also a centrist who has angered Republicans, is the measure's biggest financial backer. His campaign committee has donated $2 million to support the effort. The California Chamber of Commerce has given $720,000.
So far, voters who appear hungry for political reform agree with proponents. A poll released May 19 by the Public Policy Institute of California found 60 percent of likely voters supported Proposition 14, with 27 percent opposed.
Party leaders acknowledge an open primary will reduce their influence. They uniformly oppose the measure, creating an unusual alliance against it.
Members of the Green and the Peace and Freedom parties have begun running television ads opposing Proposition 14. Their candidates are likely to be shut out of general elections if the measure passes.
U.S. House Speaker Nancy Pelosi, D-Calif., also has come out against the measure. U.S. Rep. Zoe Lofgren, who heads California's Democratic congressional delegation, said that while the idea might look good in theory, "in the real world it will only lead to less choice and more entrenchment."
Supporters contend that the initiative will boost moderate candidates, but open elections haven't always given centrist politicians a ticket to the finals.
In 1991, Louisiana voters sent former Ku Klux Klan leader David Duke to a runoff in the governor's race. Duke's organization and funds helped him edge out the more moderate candidate in an open field of candidates, said Rob Richie, executive director of FairVote, an arm of the Maryland nonprofit Center for Voting and Democracy.
Proposition 14 is patterned after a slightly different top-two system in Washington state that has been in place since 2008.
California voters have a mixed record on open primaries. In 1996, they adopted Proposition 198, which allowed all voters to cast ballots for any candidate. It advanced the top vote-getters of each party in the primary to the general election.
It was struck down four years later by the U.S. Supreme Court for violating the parties' First Amendment right of free association.
In 2004, voters rejected an open primary measure similar to Proposition 14. Instead, they adopted a competing ballot measure that codified California's current modified-primary system, in which parties can chose to allow decline-to-state voters -- now 20 percent of the electorate -- to participate.
"People register as a member of a political party for a reason," California Democratic Party Chairman John Burton said. "It allows people to at least have an idea of what your values are."
Background on Proposition 14: