There is a perception out there that the Democrats in California are a well-oiled machine. One thing people don't notice is that the Democrats may have cost themselves not one, but two congressional districts in 2012 because their sitting congressmen didn't want to run in certain districts. Nearly 80% of Jim Costa's district was in the new CA-21. Costa decided to run in the safer CA-16. That district contained territory from several congressmen, but the plurality was from Dennis Cardoza's district. Cardoza could've run in CA-16 against Costa or run in either CA-1, which had quite a bit of his old district, or CA-21. Cardoza retired. Democrats lost both CA-10 and CA-21.
It's hard to remember now, but in 2011 Joe Baca was regarded as a strong incumbent congressman. Had he run in CA-31, it's likely that multiple Democrats wouldn't have tried for the seat and Baca would've made top two. He ran in CA-35 and Democrats lost CA-31. Today, Congressman John Garamendi decided not to rule out a run for CA-11, retiring congressman George Miller's seat. CA-11 is 47% of Garamendi's old seat and 46% of Miller's old seat. It's safe for any Democrat and Garamendi's current seat, CA-3, is potentially competitive every two years. Garamendi would face a tough foe in State Senator Mark DeSaulnier, although Garamendi did defeat him in the 2009 special election for his old district.
The bigger story, however, is that Garamendi running in CA-11 would leave CA-3 an open seat. Carly Fiorina won the district in 2010. The district moved only slightly more Democratic in 2012, so there's no reason to think the GOP wouldn't be competitive for an open seat there. Considering what happened with CA-21 and CA-31, Democrats should be afraid of this scenario.