In California, it was a cerulean tsunami.

The Democrats’ hoped-for devastating blue wave fell short in some parts of the country, but the election results inundated California’s Republican Party, leaving it as little more than a political afterthought in the nation’s most populous state.

“This was a shellacking, no question about it,” said Jim Brulte, chairman of the state Republican Party.

The scope of the Democratic victory is stunning, with the party grabbing far more than the low-hanging fruit that’s typical of a swing election.

“In much of the country, the seats Democrats won have been volleyed back and forth for years,” depending on which party was in power, said Paul Mitchell of Political Data Inc., which collects and analyzes voter information. “But in California, we didn’t have a takeover, we had an overthrow.”

Just as Orange County was long a nationwide symbol of Republican power, it became an indicator this year of how much has changed. Democrats flipped all four GOP seats in that once-conservative core.

“The midterm election losses in California were devastating,” said Republican Travis Allen, who gave up his Orange County Assembly seat to run unsuccessfully for governor. “It’s taken the Republican Party to its lowest point since the 1880s.”

Consider the numbers:

•Going into the election, 14 of California’s 53 members of the House were Republicans. Come January, that will probably drop to eight.

•In California history, Democrats have never held more than the 57 seats they had in the 80-member Assembly in 1977. They are already assured of holding that many when the Legislature reconvenes, and are leading in races that could bring the total to 60.

•Democrats lost their two-thirds majority in the state Senate in June, when Josh Newman was recalled from his Orange County seat. After flipping two Central Valley seats, they have their supermajority back with a vote to spare, meaning they can pass virtually any legislation without Republican help.

•For the third election in a row, Democrats won every statewide office, each one by double digits. Not a single Republican collected as much as 39 percent of the vote.

The reason for the GOP debacle depends on whom you ask.

For Republican Rep. Dana Rohrabacher, a 30-year veteran of Congress who lost his coastal Orange County seat to Democratic businessman Harley Rouda of Laguna Beach, it was all about the money.

“This was a bought and sold election,” he said. “I didn’t lose this vote — my district was purchased.”

Rohrabacher was outspent 3-1 by Rouda, and Democratic billionaire Michael Bloomberg of New York dumped more than $4 million into the district in the final weeks for TV ads attacking the Republican.

“The first thing that the Republican Party’s got to do is make sure our billionaires stay loyal to us in tough times,” Rohrabacher said.

But he and other California Republicans didn’t see that their districts were changing around them. In the six congressional districts Democrats have won or are winning, Democrat Hillary Clinton beat Donald Trump in 2016. Two years of Trump in the White House just hardened voters’ misgivings toward the president and his supporters.

“They were not safe Republican seats, which is why the results were as bad as they were,” said Brulte, the state GOP chairman.

The demographic changes have been dramatic, said Mitchell of Political Data. Not only is California’s Democratic-leaning nonwhite population growing, but Republicans are losing many of the wealthy suburban voters who shored up the party for decades.

“In 2009, seven of the 10 California congressional districts with the highest average household income were held by Republicans,” Mitchell said. “After the 2018 election, they only hold two of the top 10.”

The steady drip, drip, drip of Democratic gains as millions of post-election night ballots are counted shows how the state’s population is changing — and not in Republicans’ favor.

Those late ballots “don’t come from the 10 of 10 voting white homeowners who know where their stamps are,” Mitchell said. “They voted early and their votes already are tabulated.”

They come instead from groups that lean Democratic: young people, minorities, renters and others who don’t show up for every election.

But increasingly they do. Innovations in California’s election laws now register voters automatically at the Department of Motor Vehicles, send a ballot automatically to voters in an increasing number of counties, allow ballots to be counted if they’re postmarked by election day and give voters a do-over if they forget to sign a mail ballot or use an unreadable signature.

Those changes encourage more people to vote, and more of them are voting Democratic.

“I think that one of the lessons that the GOP needs to learn out of this election cycle is how to work within all the new rules,” said GOP Rep. Jeff Denham of Turlock (Stanislaus County), who was beaten by Democrat Josh Harder.

“There have been a lot of changes that caught many in the Republican Party by surprise,” Denham said.

But the party doesn’t have much time to adapt. Presidential elections always bring out more voters than midterms and with Trump on the ballot in 2020, California’s turnout could soar.

When all the votes are counted, turnout for the November election is expected to be around 63 percent, compared with 75 percent for the 2016 presidential balloting.

“Those additional voters aren’t going to be white-haired Republicans, since they always vote anyway,” Mitchell said. “It’s going to be a lot more minorities and teenagers, which should be really foreboding for Republicans.”

So what needs to change? For Allen, the failed candidate for governor who is running to replace the retiring Brulte as the state GOP chairman in February, it’s time to rebuild the structure from the ground up and make the party even more old-school Republican.

“The Republican establishment has given in to despair,” he said. “They’ve deluded themselves into thinking that the only way to be relevant is to look more like the Democrats.”

Instead, the party has to reach out to a wider range of voters with a message of lower taxes, better schools and safer communities, Allen said, adding that “Republican values are California values.”

Former GOP Gov. Arnold Schwarzenegger and Stanislaus County Supervisor Kristin Olsen, former Republican Assembly leader, want a different type of change. They’re promoting New Way California, a moderate Republican group committed to “building trust and achieving bipartisan solutions” to California’s problems.

“The California Republican Party isn’t salvageable at this time,” Olsen said in a recent opinion piece.

But for GOP Rep. Kevin McCarthy of Bakersfield, who will become House minority leader in the next Congress, there’s no reason for panic.

“One thing I know about politics: The pendulum swings back and forth,” he said. “What it also means is it won’t swing back and forth if you don’t work hard, you don’t improve your message, you don’t include more people in the process. ... So we do have our work cut out for us.”

John Wildermuth and Tal Kopan are San Francisco Chronicle staff writers. Email: jwildermuth@sfchronicle.com, tal.kopan@sfchronicle.com Twitter: @jfwildermuth, @talkopan