National Democrats, confronting mushrooming political chaos across Southern California, are pouring millions of dollars into congressional races to avert a self-inflicted disaster that could undermine their chances at taking control of the House.

After months of optimism that the state’s June 5 primary would position them to pick off seven Republican-held districts in November — a substantial down payment on reclaiming the House — Democrats are now trying to ensure that they don’t hurt themselves because of their unusually crowded slates of candidates.

With so many Democrats running, their fear is that the vote will be splintered, allowing Republicans — who have fewer candidates — to dominate some of the primaries. The party and allied groups are spending more than $4 million on just three campaigns, intervening in one contest to prop up a favored candidate; attacking a Republican from the right in another; and even reminding people not to waste their votes on “ghost candidates” who have dropped out yet remain on the ballot.