Getting a candidate into all three of those winnable districts is important for Democrats. Nonpartisan election analysis sites consider all three general election races either toss-ups or favorable to Democrats — if the party fields a candidate in November. The districts either have roughly balanced partisan leanings or have only narrowly skewed more toward Republicans than the country as a whole in recent presidential elections, according to Cook Political Report's Partisan Voter Index.
With several Democrats running in each district, both local activists and national party groups have tried to boost overall Democratic voter turnout to give candidates more votes to split. The Democratic Congressional Campaign Committee has spent millions on ground operations and ads trying to boost voter turnout and registration. The House Democrats' campaign arm has spent money to help Democrats and hurt Republicans in some districts, and even drawn the ire of some candidates for efforts to intervene or encourage Democrats to drop out.
Last week, the DCCC acknowledged the number of Democratic candidates and retirements from Royce and Issa raised the "serious risk of being shut out" in "multiple races." The retirements complicated matters because one or more Republican running for an open seat could take a larger share of the primary vote than one incumbent would.
House Republicans' campaign arm has also spent money in the three districts' primaries. Last week, the National Republican Congressional Committee kicked off what it called a six-figure ad buy to motivate Trump voters in the 39th, 48th and 49th Districts.
A Republican familiar with the races argued lockouts would be "catastrophic" because of Democrats' efforts and spending to ensure a party candidate makes it through Tuesday's primaries.
In the 39th District, former state Assemblywoman Young Kim appears to have the edge among Republicans. Leading Democrats Gil Cisneros — whom the DCCC backs — and Andy Thorburn previously reached what Vigna called a "ceasefire agreement" to stop attacking one another ahead of the primary. The DCCC has tried to prop up Cisneros, a Navy veteran and education advocate, and damage Republicans Shawn Nelson and Bob Huff, who could play spoiler to the Democrats.
Rohrabacher is expected to win one of the general election spots in the 48th District. Democrats have tried to hit GOP former California Assemblyman Scott Baugh and help businessman Harley Rouda, their preferred candidate in the race over stem cell researcher Hans Keirstead.
In the 49th District, leading Republicans include state Assemblyman Rocky Chavez and former state Assemblywoman Diane Harkey. Contenders on the Democratic side include clean energy advocate Mike Levin, Marine veteran Doug Applegate and former State Department official Sara Jacobs. National Democrats have run ads criticizing Chavez.
A local advocacy group called Flip the 49th has worked to increase Democratic turnout in the district, using canvassing and phone banks to urge infrequent Democratic voters to cast ballots. Terra Lawson-Remer, the group's chair, said it has not backed a particular candidate but believes the efforts have reduced the chances of a lockout in the district.