David Wasserman for the Cook Political Report:

Republicans badly need a few lucky breaks to hold their House majority in November. So far in 2018, it’s been the opposite story — from an unfriendly new Pennsylvania map to Speaker Paul Ryan’s retirement and bleak special election results. But with five weeks to go before California’s June 5 primary, Democrats are at risk of squandering several seats that would otherwise appear to be golden pickup opportunities.

Democrats’ path to a majority depends on California more than any other state: they have excellent chances in seven GOP seats that Hillary Clinton carried in 2016, and a few more could be long shots in a wave. But in at least four districts, Democratic over-enthusiasm has produced crowded fields that could lock Democrats out of the fall race altogether.

Under California’s unorthodox “top two” primary system — first implemented in 2012 — all candidates appear on the same June primary ballot and the top two finishers, regardless of party, advance to a November runoff. In 2012, catastrophe struck Democrats when their top candidate in the new 31st CD, Pete Aguilar, took third place in the primary behind two Republicans, locking them out of a highly winnable race (Aguilar won the seat in 2014).

The same fate could befall other Democrats in 2018. In the 39th, 48th and 49th districts — all Orange County GOP seats that voted for Clinton — the “blue wave” has generated throngs of viable Democratic candidates in districts where GOP voters traditionally make up a majority of the primary vote. And while Democrats have struggled to break out of their packs, there are at least two viable Republican candidates on the ballot in each of those races.