DEMOCRATIC enthusiasm—defined not just by voter turnout but also by the number of candidates—has defined this year’s primary elections, as it did last year’s special elections. That is not unusual; Republican enthusiasm defined the 2010 midterm elections—Republicans then, like Democrats now, hold neither the White House nor either congressional chamber. Mid-term elections usually go badly for the party in power. But enthusiasm has its downsides, as may become evident in California’s primary on June 5th.

All told, Republicans hold 23 congressional seats in districts that Hillary Clinton carried two years ago. Seven of them are in California—more than in any other state. To reclaim the House, Democrats need to pick up 24 seats. The road to their majority thus runs straight through California. If the state had ordinary primary elections, there would be no problem: on June 5th Republicans would choose their candidates, Democrats theirs, and the two would face off in the general on November 9th.

California, however, has a “jungle primary” system, in which all candidates appear on a single ballot, and the top two advance to the general, regardless of party (hence the 2016 senate general election, which featured two Democrats: Kamala Harris and Loretta Sanchez). Unfortunately, Democratic enthusiasm has produced not just crowded candidate fields—which are not inherently a problem: imagine a liberal district in which Elizabeth Warren, a prominent senator, faced off against three dullards—but fields crowded with equally viable candidates. The fear is that in districts where Democrats hold a slight edge in total voters, several Democrats could more or less evenly split, say, 52% of the vote, while two Republicans split 48%. The two Republicans would finish atop the ballot and advance to the general, despite Democrats winning more votes.

This fear is especially acute in five districts. Dana Rohrabacher has held his seat in Orange County for almost 30 increasingly scandal-plagued years. He has a strong Republican challenger, Scott Baugh, while the fight among five Democrats is turning increasingly nasty. Also in Orange County, Young Kim seems to have consolidated Republican support to succeed Ed Royce, who is retiring, while four Democrats battle it out on the other side. Further down the coast, Darrell Issa is also retiring. Here the chances of a lockout cut both ways: the field has four Democrats and eight Republicans, with no clear front-runner on either side.

Just east of Mr Issa’s district, Duncan Hunter is running for reelection, despite being under federal investigation; he has drawn a pair of Republican challengers and three Democratic hopefuls. This seat is more solidly conservative than the others (Donald Trump won it by 15 points), but Democrats would still rather be on the ballot in November. And in central California, Jeff Denham has drawn a single Republican challenger, while Democrats have a choice of six candidates. Mr Denham, a moderate currently fighting to pass immigration reform over the wishes of his caucus’s powerful hard right, will probably finish first. But Democrats could find themselves locked out if his challenger siphons off enough disaffected Republicans.

The jungle-primary system, introduced in 2010, was supposed to encourage moderation. It seems instead to have produced confusion and gamesmanship. Democrats have begun intervening in the races—endorsing (rare in primaries), pressuring less viable candidates to drop out and airing ads attacking Republican candidates from both left and right, in an effort to direct Republican support toward outcomes favourable to Democrats. Sometimes that outcome is a Republican spot on the ballot: progressives, for instance, would much prefer to see Gavin Newsom (pictured), the front-runner for governor, face a Republican in November than Antonio Villaraigosa, a well-funded moderate likely to finish second. Mr Villaraigosa, the thinking goes, could beat Mr Newsom, while a Republican’s chances of winning statewide in California are pretty slim.