California voters eliminated traditional party primaries in 2010, replacing them with the top-two system at the urging of Arnold Schwarzenegger, then the Republican governor, and other moderates. They argued that a primary open to all voters would lead to political moderation by candidates who, in theory, could not win by appealing only to the base of their party.
The voter initiative was vigorously opposed by Democratic and Republican leaders who warned that it would force candidates to invest time and money in debilitating primaries and runoffs, and could lead to general elections where a major party would be kept off the ballot.
It is far from clear whether the top-two system — which has been adopted in some form by four states so far — has lived up to its promise of good government reform, or created the kind of one-sided general elections that so concerned party leaders. That happened in California in a congressional race in 2012, in which the general election choice was between two Republicans in a district with a Democratic edge, and in last year’s Senate race, in which no Republicans were on the general election ballot.
Indeed, in the midst of this heated battle, it is still a matter of dispute whether the change has made much of a difference at all. Thad Kousser, a professor of political science at the University of California, San Diego, suggested that so far it has not. “We haven’t seen big shifts in the Legislature,” he said. “It hasn’t changed California politics radically.”
The stakes now are particularly high. Mr. Issa’s seat is one of seven congressional seats that Democrats are targeting to flip in California this year. It is one of two where the Republican incumbent is not running again, which otherwise would have likely assured a straight runoff between the sitting Republican and a Democratic challenger. (The other is Representative Ed Royce’s to the east).
Democratic leaders are moving to winnow the field — steering contributions to favored candidates, moving to award the state party endorsement to one person and warning candidates who might have embarrassing chapters in their professional or personal lives of the kind of scrutiny and political attacks that come with entering public life.
But any attempt by the party to interfere is complicated by the strains between establishment Democrats and the more liberal wing of the party, which have been on particular display in this state. Democratic leaders are wary of coming across as old-school bosses stampeding the concerns of grass-roots activists.
“My job is not to tell people they can’t run,” said Eric C. Bauman, the state Democratic leader, who has been trying to trim the field to, ideally, he said, two Democrats. “It’s not to push people out of races. But to try to help good candidates look to see if they have other options they could run for and make an equally important contribution.”
Mr. Bauman said the current circumstances had confirmed what have been his criticisms of the top-two method from the beginning.
“That denies people an opportunity to vote for a candidate who represents their interests,” Mr. Bauman said. “It creates that danger that we could end up with so many Democrats that we split the vote so badly that we get aced out of a spot in November.”
Given California’s status as the leading edge of Democratic efforts to take back the House, the situation is a top concern of national Democrats.
“We plan on having a Democrat running in both races this November,” said Daniel Sena, the executive director of the Democratic Congressional Campaign Committee. He said the committee was hoping this could be accomplished quietly, but was prepared to move more aggressively — such as by steering financial contributions to favored candidates if it looked like Democrats might be running themselves to defeat.
“We still reserve the right to get involved,” he said. “Right now we are preserving the right to invoke military options and spend money as we see fit.”
This is not only a problem for Democrats. There are four Republicans running in Mr. Issa’s district and six in the Royce district. But Republicans have a registration edge in both districts, making it less of a threat for them, officials in both parties said.
When the top-two system was first put on the ballot, leaders of both parties opposed it. And in 2016, after two Democrats emerged from a crowded primary to run for the United States Senate seat eventually captured by Kamala D. Harris, a Democrat, one of the losing Republicans, Tom Palzer, a retired city planner, launched his own initiative campaign to repeal the system.
“When you go to the ballot box and two people from one party, you don’t have a choice,” he said. “I don’t care if it’s two Republicans, two libertarians. We need healthy parties and we need competitive parties in the State of California. I’m a Republican. If the top two were Republican, that wouldn’t be right.”
But other Republicans, after watching the problems it has caused Democrats have reconsidered their position, and opposed an effort to persuade the state party to back Mr. Palzer’s effort. They argued to party members that repealing it would “rescue Democrats from their brutal and costly infighting.”
Jim Brulte, the state Republican chairman, said it was unclear what the party would do if Mr. Palzer’s initiative gathers enough signatures to appear on the ballot.
“When the top two was on the ballot, every political party in California opposed it,” he said. “The party has not taken a position on a repeal and will not take a position on it unless there’s an actual initiative.”
Christian Grose, a professor of political science at the University of Southern California, who is writing a book on the system, said that by his count, there had been 159 congressional votes in California since the system was put in place — and the outcome that party leaders had feared happened in only one congressional race. “It doesn’t mean it couldn’t happen,” he said. “I didn’t want to overstate it. It seems like empirically we just haven’t seen that happen.”