Republicans took a strong first step in the primary election toward grabbing back at least some of the seven California congressional districts that Democrats flipped in 2018.

With millions of late-arriving mail ballots still uncounted from Tuesday’s election, Republicans have so far combined for a majority of the votes in six of those seven districts. GOP candidates have solid leads in two of them.

The results show just how tough it’s going to be for Democrats to hang onto everything they won two years ago, when California victories helped them take back the House, said David McCuan, a political science professor at Sonoma State University.

“California is a blue state, but parts of it are purple,” he said. “It will be a difficult road for the Democrats to run the table again this year.”

With all seven of those districts targeted by Republicans eager to regain control of Congress, Democrats may have to lower their expectations, McCuan said.

“In November, a good night for Democrats may be winning four of those seats,” he said.

Republicans were gleeful over Tuesday night’s results.

In Orange County, GOP candidates were winning a majority of the votes in three of the four targeted districts. The primary numbers were a welcome salve to the embarrassment of the 2018 election, when Democrats swept to victory in all the county’s congressional districts.

“On Tuesday night, the waters of the blue wave began to recede ... and Republicans showed that we will take back Orange County,” Fred Whitaker, chairman of the county GOP, said in a statement. “The pathway to restoring the House of Representatives is through Orange County.”

That may be more confidence than the numbers warrant. Of those contested seats, only former GOP Assemblywoman Young Kim of Fullerton holds an actual lead over a Democratic incumbent. After losing a close race to Rep. Gil Cisneros in 2018, she leads him in the primary rematch, 51% to 48%.

In a two-person contest on the border of Orange and San Diego counties, Democratic Rep. Mike Levin of San Juan Capistrano was on top of Republican Brian Maryott, 53% to 47%. Democratic Rep. Katie Porter of Irvine had 48% of the vote against six GOP challengers, and Rep. Harley Rouda of Laguna Beach was leading Orange County Supervisor Michelle Steel, 44% to 37%.

Collectively, however, Republicans were outpolling Democrats in both Porter’s and Rouda’s districts.

Until 2018, Orange County’s seats had been Republican for decades. GOP leaders argue that even with a competitive Democratic presidential primary, the fact that more Republicans turned out than Democrats is a positive sign for the enthusiasm of GOP voters.

Republicans made clear the type of races they plan to run in Orange County and throughout the state.

In congratulating Kim on her showing, Minnesota Rep. Tom Emmer, chairman of the National Republican Congressional Committee, said that in November “voters will reject Gil Cisneros and his highly unpopular socialist agenda.” He likewise dismissed Levin as progressive New York Rep. Alexandria Ocasio-Cortez’s “acolyte.”

In the Central Valley district where GOP Rep. Devin Nunes of Tulare got 59% of the vote, Emmet called Phil Arballo, who finished well behind the leader, “a socialist Democrat who wants to do nothing but impeach President Trump and abolish private health insurance.”

Tuesday’s worst Democratic showing in a 2018 flip race was near Nunes’ district. Former GOP Rep. David Valadao held a 53% to 36% lead over Rep. TJ Cox of Fresno.

In 2018, Cox beat Valadao by fewer than 1,000 votes in a race that wasn’t decided until a month after the election.

The other Democrat who flipped a Central Valley seat in 2018, Rep. Josh Harder of Turlock (Stanislaus County), finished first in his primary. But he and two other Democrats were collectively being outpolled by three Republicans.

Cox tried to put the best face on his showing, saying in a statement that “we’re still in this fight as underdogs” and that “this race will be decided in November.”

His assertion that Democrats will do better in the fall has been echoed by party leaders, who were quick to note that voters in California primaries tend to be older, whiter, wealthier and more Republican than those who turn out for elections in November.

In 2018, for example, Valadao steamrolled Cox in the primary, 63% to 37%, but Cox came back for the narrow victory in the general election. In all seven of the districts that flipped two years ago, Republicans saw their share of the vote decline in November.

But that GOP primary strength could come back to bite the Democrats in a May 12 runoff election to decide who will serve the remaining term of former Democratic Rep. Katie Hill of Santa Clarita (Los Angeles County), who resigned in November after admitting to a sexual relationship with a campaign aide.

Democratic Assemblywoman Christy Smith finished first in the primary contest for the November election with 30% of the vote and was followed by Republican businessman Mike Garcia with 26%. Smith did even better in the special election, finishing ahead of Garcia, 34% to 27%.

Since Smith didn’t receive a majority, however, the pair will meet in the May runoff, which will be the only contest on the ballot. Republicans tend to be more likely to vote in such low-turnout elections, meaning Garcia could have the inside track on filling out the remaining months of Hill’s term.

John Wildermuth is a San Francisco Chronicle staff writer. Email: jwildermuth@sfchronicle.com Twitter: @jfwildermuth