LOS ANGELES (AP) — When Rep. Gary Miller announced in February he would retire from Congress, it was hardly a surprise. The surprise was that Miller was a member of Congress.

The wealthy Republican’s unexpected 2012 victory in a Southern California district President Barack Obama carried by 17 points was largely attributed to the quirks of the state’s primary election rules, which get their second test June 3.

The system known as “top two” or, as detractors call it, the “jungle primary,” was one of two remedies California voters ushered in amid widespread dismay with gridlock in Sacramento and Washington. In Miller’s case, so many Democrats ran in the primary two years ago they effectively canceled each other out, sending two Republicans to the runoff in a heavily Democratic district.

In California’s relatively new primary system, the top two vote-getters move on to the general election regardless of their party affiliation. The prospect of a sparse turnout in June — some suggest it could be a record low — compounds what already was a risky political landscape under the revamped rules.

Campaigns must “take into account more variables and wild cards than we’ve ever had to before,” said Democratic consultant Roger Salazar, whose stable of former clients includes former vice presidential nominee John Edwards and former California Gov. Gray Davis.

It “creates a lot more opportunity for dark horse candidates to sneak in there,” he said.

The idea behind the top-two plan was to elect more politically moderate officeholders, but some of the results the first time around were head-scratching: Voters in several legislative and congressional districts in 2012 ended up with two Democrats or two Republicans on the November ballot, with no other choices.

Meanwhile, the new system appeared to do little, if anything, to lure more voters to the polls. A recent study of primary elections by the Public Policy Institute of California found its first run in 2012 “failed to produce the increase in turnout that many had hoped for, and there is little evidence that open primaries in other states have fared any better.”

In a traditional primary election, voters from a political party choose a nominee from within their own ranks who advances to the November ballot. It would be the same for voters registered to minor parties, such as the Greens or Libertarians. In general, voters in November would choose from a list of candidates representing various political parties.

By comparison, the top-two primary is a free-for-all.

All candidates appear together on a single ballot. Voters are permitted to pick anyone on the list, but only the two attracting the most votes advance from the primary to November.

Miller, first elected in 1998, migrated into the Democratic-leaning district and beat another Republican in November 2012 after several Democrats split, and in effect diluted, the Democratic vote in the primary. That allowed the two Republicans to slip through to the runoff, even though Democrats hold a 7-point voter registration edge in the district, which is anchored by the city of San Bernardino.

“There’s something troubling about a November election in which a voter cannot choose between parties,” political scientist Jack Pitney, who teaches at Claremont McKenna College, said in an email. “Another problem is the absence of third parties. In most cases, the system now keeps them off the ballot in November.”

The Green, Libertarian and Peace and Freedom parties are pushing a lawsuit in state court that argues the top-two contest essentially bars third-party candidates from the November ballot. The case is on appeal.

In their legal quest to change the system, the third parties have found common purpose with the two main political parties. None of them like it, raising the possibility of a future repeal effort.

“There ought to be a clean and easy primary for Republicans to choose their own nominee, same for the Green Party, same for the Democratic Party. Then you have a vivid contrast in points of view,” said Republican National Committee member Shawn Steel. “The experiment failed.”

Bob Mulholland, a longtime adviser to the California Democratic Party, expects voters to do away with the system eventually.

In the other major reform, California voters stripped the power to draw district lines from state lawmakers in 2008, instead establishing an independent commission to sculpt legislative and congressional boundaries.

The safe seats and rubber-stamp elections that once predominated are gone, and California is home to some of the most competitive legislative and congressional contests in the country.

While its effect on the political climate remains less clear than that of independent redistricting, there is little doubt the top-two primary has produced surprises that would have seemed improbable before the changes.

For example, long-serving Rep. Pete Stark was dispatched from his San Francisco Bay Area congressional seat in 2012 by a fellow Democrat nearly 50 years his junior, Eric Swalwell. The challenges of the revamped system can be witnessed this year in the 33rd Congressional District, where Democratic Rep. Henry Waxman is exiting after four decades on Capitol Hill. Eighteen candidates are on the June primary ballot, including 10 Democrats.

With such a crowded field of Democrats, it’s possible a Republican could emerge as the top vote-getter in the strongly Democratic district and land a spot in the November runoff.

“I don’t think anyone has a clue who is going to win that thing,” said consultant Andrew Acosta, who is advising one of the Democrats, radio host Matt Miller. “You could throw a dart at a lot of these races.”

In the governor’s race, the system could be working against the kind of candidate it was designed to help, Republican centrist Neel Kashkari.

Independent polls show Kashkari, a former investment banker and U.S. Treasury official, is barely making an impression with voters despite months of campaigning. That suggests the far better known governor, Democrat Jerry Brown, is sucking up the middle-ground vote in the primary.

In a traditional Republican primary, Kashkari’s turf would be anyone to the political left of his main GOP rival, tea party favorite Tim Donnelly, a state assemblyman. But in the new primary format “he’s getting squeezed from both sides,” said Democratic consultant Bill Carrick, who is not involved in the governor’s contest.

“In the top-two, he has another competitor, Jerry Brown, who also has appeal to moderates,” Carrick said. For Kashkari, “It’s a tough dilemma.”