YORBA LINDA, Calif. — Celina Estrada and Sam Zapata weren’t even born when Republican Ed Royce was first elected to Congress in 1992. Yet a year before the 2018 elections, the two students spent a recent evening knocking on doors in the hills of Orange County, California, to support the vulnerable congressman.

Royce hasn’t had a close race in years. In 2016, he won with 57 percent and outspent his Democratic opponent, $3.7 million to $77,000. This cycle, however, inspired to counteract the effects of a Donald Trump presidency, five of his Democratic challengers had over $100,000 in their campaign accounts at the end of September, and two of them are self-funders.

Hillary Clinton was the first Democratic presidential nominee to win Orange County since 1936, sparking interest from Democrats in defeating a handful of GOP incumbents, including Royce and Reps. Darrell Issa, Dana Rohrabacher, and Mimi Walters. But Republicans have already started establishing a ground game, providing defense to some of their most vulnerable incumbents.

Gaining a seat or two in Southern California might leave Democrats short of the 24 they need to gain nationwide for a House majority. But defeating three incumbents — or a clean sweep of Orange County — would put the party in prime position.

Royce (who had $3.4 million in the bank as of Sept. 30) will likely benefit from a flood of television ads from outside groups late next year, but he’s already getting unprecedented ground support from the Congressional Leadership Fund, a super PAC aligned with Speaker Paul D. Ryan.