Indeed, in the middle 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 had 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 probably 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.”