But in some areas, Democratic leaders have concluded it is worth enduring backlash to help a prized recruit or tame a chaotic primary field.
They are moving most aggressively in California, where the state’s nonpartisan primaries present a unique hazard: State law requires all candidates to compete in the same preliminary election, with the top two finishers advancing to November. In a crowded field, if Democrats spread their votes across too many candidates, two Republicans could come out on top and advance together to the general election.
There are at least four races in California where Democrats fear such a lockout, including the 39th Congressional District, where in addition to Mr. Cisneros and Ms. Tran there are two other Democrats running: Sam Jammal, a youthful former congressional aide, and Andy Thorburn, a wealthy health insurance executive who is backed by allies of Senator Bernie Sanders, independent of Vermont. The district is among the most coveted for Democrats nationwide — a seat left open by the retirement of Representative Ed Royce, a popular Republican, in an area Hillary Clinton won by about 8 percentage points.
National Democrats may also intervene in the Southern California districts held by Representatives Dana Rohrabacher and Jeff Denham, where multiple Republicans and Democrats are running, and in the seat held by Representative Darrell Issa, a Republican who is retiring. Voters receive mail-in ballots starting in early May, making the next few weeks exceptionally important.
House Majority PAC, a heavily financed Democratic group that spends millions in congressional elections, recently polled all four races and has been conducting digital surveys that simulate the complicated California ballot, according to people briefed on the group’s strategy. The super PAC has run ads in California in the past when Democrats have faced disaster in primary season.