California is known as a deep-blue Democratic state, one where Republicans are little more than an afterthought. But a map of voter registration by county shows just how rural California Republicans have become.

There’s still plenty of red on that map, but it’s almost all concentrated in the far north and the Sierra, home of tiny counties with more trees than people.

Since the 2018 midterm elections, San Luis Obispo and Orange counties have flipped from Republican to Democratic in registration. Both Kern and Trinity counties are a single percentage point from turning blue.

The 35% of voters who identified with the GOP in 1999 fell to 24% in the most recent registration numbers released in February, and Democrats hold every statewide office and supermajorities in both the Assembly and the state Senate.

Even so, Democrats already are planning registration drives, using President Trump’s unpopularity in the state as a way to bring more unregistered people and independent voters into the party and solidify the gains Democrats made in Congress and the Legislature last year.

“The bottom line is that 2018 was a giant wave, but what is even more exciting is that the enthusiasm we saw in 2018 hasn’t died down one bit,” said Andrew Feldman, spokesman for Hold the House, a California political action committee chaired by former Democratic Rep. Mike Honda of San Jose. “People across California understand what is at stake and not only want to do everything they can to hold onto the gains that we made it 2018, but expand into more areas across the state.”

The GOP’s isolation in the state is staggering. With the recent addition of Orange and San Luis Obispo counties to the Democratic fold, the party now has a wall of blue up the California coast, stretching from the Mexican border to the Republican’s lone coastal outpost of Del Norte County, population 27,828, the last stop before Oregon.

The Republicans’ biggest stronghold is the Central Valley’s Kern County, followed by neighboring Tulare County. But Kern is only the 11th-largest county in the state by population and Tulare is No. 18. Those two, along with Placer County (22) and El Dorado County (29), are the only places in the top half of California’s 58 counties where Republicans hold a registration edge.

For many people in the sprawling metropolitan regions of the Bay Area and Southern California, the diminutive size of many of the state’s GOP-friendly rural counties is hard to imagine.

Sutter County, the 37th-largest, has 96,807 residents, about 10,000 fewer than Daly City. Glenn County (48) is a bit smaller than Benicia, while Sierra County (57) is almost the same size as Yountville, California’s 771st-largest city.

Physical size doesn’t make much difference. Inyo County, where Republicans have control, is the second-largest county in the state at 10,181 square miles, while San Mateo County, where Democrats hold a commanding 50% to 15% lead in registration, is No. 56, at 520 square miles.

But San Mateo County has 727,209 residents, compared with 17,987 for Inyo County.

The Democrats’ domination of large, unbroken swaths of the state is an ongoing problem for the GOP, said Bryan Watkins, political director for the California Republican Party.

“It’s something we’ve got to focus on,” he said. “We need a different approach in the bigger counties than we take in rural areas. We’re not talking about ‘one size fits all.’”

The party’s geographic woes don’t mean it’s impossible for Republicans to win in Democratic-leaning cities and counties. Republican Kevin Faulconer has been elected twice as mayor of San Diego, and last month, the GOP’s John Lee’s won a seat on the Los Angeles City Council, despite the Democrats’ 54% to 12% lead in the city.

“The best thing we can do is just keep showing up,” Watkins said. “We tend to think in two-year election cycles, but in bigger cities it’s going to take a longer time” to show results.

Republicans have plenty of places where they can test their strategy. In California’s largest cities, the GOP is already an endangered species. Republicans make up 12% of the registered voters in Los Angeles, 16% in San Jose, 15% in Sacramento, 6% in San Francisco and 4% in Oakland.

The biggest city where Republicans hold sway? That would be Huntington Beach in Orange County, California’s 24th-largest city, with 200,641 residents.

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