That’s where another high-stakes special election will take place on July 12, to fill the seat of former Democratic Rep. Jane Harman. Lacking a sexy Mediscare plot, it isn’t getting much national play. But the bottom line is that Democrats are struggling to hold on to a blue seat, offering a vivid preview of the extraordinary economic vulnerabilities the party faces going into a presidential election. There are lessons here, just as potent as those from New York.

California’s 36th district is a sprawl of coastal communities near Los Angeles, heavily gerrymandered to favor Democrats, who outnumber registered Republicans 45% to 27%. Mrs. Harman routinely took more than 60% of the vote.

California recently adopted a “jungle primary” system that requires all candidates from all parties to run on the same ballot, with a runoff if no candidate receives 50% of the vote. The district is so reliably Democratic that the only question on the media’s mind in the first round of May voting was which of the five Democrats (out of 16 ballot candidates) would go to a July runoff.

The Democratic frontrunners, after all, had not just a voter advantage, but huge name recognition and statewide political support. Los Angeles City Councilwoman Janice Hahn hails from a family dynasty—her brother was mayor of Los Angeles and her father was a county supervisor for 40 years. Her biggest rival was California Secretary of State Debra Bowen. In its endorsement of Ms. Hahn, the Los Angeles Times bothered to mention one of the six Republicans on the ballot, noting Redondo Beach Mayor Mike Gin might have an outside shot, given his “rare breed” status as a “liberal Republican.”

Would-be California Congresswoman Janice Hahn (right) gets a lecture on foreclosure.

When the dust settled on May 17, Ms. Hahn had claimed 24% of the vote. In second place, with 22%, and claiming the runoff spot, was a GOP candidate that most of the media had never mentioned: Craig Huey. A businessman, Mr. Huey had poured personal resources into a strong mail, TV and radio strategy, and garnered the endorsements of respected California Republicans.

But what really resonated with voters was Mr. Huey’s laserlike focus on the economy and jobs. As Ms. Hahn and Ms. Bowen competed on who had a more progressive environmental record, Mr. Huey banged relentlessly on California’s 12% unemployment rate, job-killing regulations, and record deficits. As the two Democratic heavyweights traded barbs over who had taken more “oil money,” Mr. Huey touted his plans for reviving growth.

His strong finish, and continued appeal, ought to be scaring the geewillikers out of Democratic leaders. Though the 2010 election was eight months ago, conservative enthusiasm for change remains sky high. Mr. Huey’s economic message has resonated with Republicans within the district and tea party leaders without. Activists have decamped to the 36th to walk precincts and make calls for Mr. Huey. Special-election turnouts are notoriously low, though in this case Democrats proved particularly uninspired. Some 25% of registered Republicans turned out in the primary, compared to 18% of Democrats—a significant factor in Mr. Huey’s second-place finish.

More worrisome to Democrats ought to be the appeal of Mr. Huey’s economic focus to independents, and even some Democrats. The area has its share of blue-collar workers, who have felt the brunt of the economic crisis. California Gov. Jerry Brown’s proposals to significantly raise taxes—even as state Democrats refuse to accept real spending reform—have sharpened discontent. Mr. Huey has blamed economic malaise on Democratic policies and pounded his opponent for offering no solutions.

This week’s word is that internal Democratic polling numbers show Ms. Hahn with a mere five point lead going into the runoff. Her cushion may be bigger, and Democrats might have put out the low number to frighten their base into turning out. But even this suggests a surprising level of Democratic worry in a seriously blue California district.

Fear is the party’s sole weapon. Lacking a good-news economic story, or a plan for turnaround, Ms. Hahn has been desperate to make the race about social issues. Mr. Huey is pro-life and has run websites that aid Christian voters. The Democrat is playing the “right wing extremism” card, hoping it will frighten in a district that voted 57% against the California ban on gay marriage. Even Medicare hasn’t gotten much attention.

Mr. Huey has largely refused to engage in the social fight, though this week an outside PAC called Turn Right USA ran an utterly offensive Web ad, comparing Ms. Hahn to a stripper with connections to gang members. Mr. Huey has condemned the spot, but Hahn supporters are making sure it dominates the headlines, hoping to overshadow Mr. Huey’s economic message.

Ms. Hahn’s strategy is to talk about anything but the economy. And that’s her problem. The lesson of California for both Republicans and Democrats is that the candidate willing to talk about growth and jobs is a candidate the electorate is willing to hear.

Write to kim@wsj.com