With just 100 days to go before the Nov. 6 election, Republicans are preparing to pour millions of dollars into tight congressional races in California and across the nation in an effort to block Democrats from flipping the 23 seats they need to take control of the House.

Democratic candidates in California have shown a strong ability to raise money — in some cases, they’ve pulled in more than Republican incumbents. So on Thursday, the Republican National Committee announced the transfer of $4 million each to the party’s congressional and Senate campaign committees for use in the midterm elections.

“We have the resources to protect and strengthen our Republican majorities in Congress,” Ronna McDaniel, the committee’s chairwoman, said in a statement. She added that she was confident the party can “elect more Republicans so President Trump can continue to deliver for the American people.”

Trump himself has agreed to send money from his 2020 re-election campaign to nearly 100 GOP candidates in what campaign officials said was the first round of contributions.

The announcements came just days after campaign financial reports showed that Democratic challengers have outraised their GOP opponents in 10 of California’s 14 Republican-held congressional districts.

Those numbers are a flashing red warning light for the Republican national leadership, said Tony Quinn, a former GOP consultant who is now senior editor of the nonpartisan California Target Book, which focuses on elections in the state.

“Republicans have no choice but to pump money into California,” he said. “Given the way the rest of the country looks, they can’t hold the Democrats under 23 (seats) nationally if they lose four or five in California.”

A July 25 Quinnipiac University poll found Democrats with a 51 percent to 39 percent lead in a generic House poll, where registered voters were asked only if they supported a Democrat or Republican for a House seat.

The Cook Political Report, a nonpartisan guide to House, Senate and governors’ races across the nation, said in an article Friday that Democrats “remain substantial favorites for House control,” largely because Republicans have to defend 42 open seats and face fundraising difficulties in many of those races.

The report lists one California seat, the district in Orange and San Diego counties held by retiring GOP Rep. Darrell Issa, as “leans Democratic,” and four other GOP seats in the state as “toss-ups.”

The only question about the GOP money push is, “What took them so long?” said Thad Kousser, a political science professor at UC San Diego.

“The stakes are existentially high for the Trump administration,” he said. “Usually a midterm election is about a president’s legacy or programs, but this election could be about Trump’s political survival.”

A Democratic-held House would give its members subpoena power and mean “investigation after investigation after investigation,” Kousser said.

That prospect isn’t lost on Trump. Speaking Thursday at a recently reopened steel plant in Granite City, Ill., the president called on the crowd to back GOP members of Congress.

“You’ve got to vote Republican, folks,” he urged. “You’ve got to vote Republican.”

That might not be a plea that carries much weight in California, where Trump received only slightly more than 31 percent of the vote in 2016 and lost to Hillary Clinton in seven GOP-held congressional districts.

“What Trump can do for California Republicans is hide his face and show them the money,” Kousser said. “There may be parts of the country where he can help candidates by holding a rally and standing next to them, but California isn’t one of them.”

Trump isn’t the only GOP politician with plenty to lose if Democrats take over the House. According to Politico, Bakersfield GOP Rep. Kevin McCarthy, a candidate to replace retiring Wisconsin Rep. Paul Ryan as speaker, gave $6 million in campaign cash to Protect the House, a political action committee he founded in February with Vice President Mike Pence.

Targeted House Republicans including Reps. Mimi Walters of Irvine (Orange County), Devin Nunes of Tulare, Jeff Denham of Turlock (Stanislaus County) and David Valadao of Hanford (Kings County) all pledged $100,000 or more to the committee.

A “blue wave” that sweeps Democrats into control of the House not only means McCarthy won’t become speaker, but also costs GOP members of Congress the political clout that comes with being in the majority.

It also could give Democrats a chance to quash many of Trump’s efforts to change environmental policies and regulations he argues have hurt both businesses and the national economy.

That’s one reason billionaire casino owner Sheldon Adelson and his wife, Miriam, gave $30 million earlier this year to the Congressional Leadership Fund, a super PAC “exclusively dedicated to protecting and strengthening the Republican majority in the House of Representatives.”

Other major donors include oil companies Chevron, Valero, Occidental and Conoco Phillips, development companies and business people including San Francisco’s Charles Schwab and William Oberndorf.

The group spent $6.2 million to defeat Georgia Democrat Jon Ossoff in a congressional special election June 2017 and another $3 million in an unsuccessful effort to prevent Democrat Conor Lamb from winning a Pennsylvania special election in March.

The fund had $71 million cash on hand as of June 30. At least some of it is likely to find its way to GOP candidates in California.

Even with money, Republicans will have a tough time hanging on to all their California congressional seats, especially if Democrats succeed in making the election about Trump, who remains deeply unpopular in the state, said Quinn of the California Target Book.

But that doesn’t mean it will be an easy road to victory for Democratic challengers, he added.

“It’s still too early to say much about the races in California,” Quinn said. “But the Republican incumbents all survived the 2016 election, when Trump got 31 percent.”

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