FULLERTON, Calif. — Democrats who cheered the retirement announcements of Reps. Darrell Issa and Ed Royce last week are sobering up to a new fear: A potential nightmare scenario in which no Democratic candidate ends up on the November ballot in either seat, dealing a blow to the party’s efforts to retake the House.

The problem is California’s unusual, top-two primary system, where the top two vote-getters regardless of party affiliation advance to the November general election.

Prior to the retirement announcements, Democrats had been pounding for months on Royce and Issa, yoking the two vulnerable Republicans to a president loathed in this heavily Democratic state. But with no GOP incumbent in either race — and with Democratic candidates threatening to splinter their party’s share of the vote — Democrats now face the prospect of getting scrubbed entirely from the November ballot [.....]