Donald Trump vowed to win California. The Republican nominee is now so weak there he could drag four GOP congressmen down with him.

Rep. Darrell Issa, the high profile former chairman of the Oversight and Government Reform committee, is now in trouble; so are Rep. Jeff Denham; Rep. Steve Knight and Rep. David Valadao.

Trump's boast about putting California in play always smacked of wishful thinking. The state hasn't voted Republican for president since 1988 and is among the most liberal in the country.

But Republican insiders there and in Washington say that Trump's unpopularity in ethnically diverse California is now threatening to crater their party down ballot — hurting members who were previously presumed safe from top of the ticket headwinds.

"The question with Trump is how low does he go?" said Bill Whalen, a political analyst at the Hoover Institution, a conservative think tank in northern California.

"If he finishes below 35 percent, that puts him in the company of the likes of Alf Landon and Al Smith," Whalen added, "and it raises the question of undertow for Republicans down-ballot."

Landon, a Republican and the governor of Kansas, pulled 31.7 percent in California against President Franklin Roosevelt in 1936. Smith, a Democrat and the governor of New York, garnered 34.2 percent against GOP nominee Herbert Hoover in 1928.

Republican sources say that Trump has been tracking below 30 percent in California in private polling, underperforming the last two GOP nominees by more than 7 percentage points.

Hillary Clinton is sitting around 55 percent in the state, less than President Obama's winning numbers in 2008 and 2012, but not to a degree that matters. Additionally, Trump poses another challenge.

If the Democratic nominee wins the election and the presidential race is called before 11 p.m. Eastern Time, when the polls close in California, Republicans there who did not vote earlier in the day could decide it's not worthwhile to show up. The consequences for endangered House Republicans could be devastating.

"The four vulnerable California House Republicans are in serious jeopardy," a veteran GOP insider in the state said, on condition of anonymity in order to speak candidly.

The extent of each four members' vulnerability is a matter of some debate. But that three are receiving outside assistance from outside Republican sources is an indicator that the party is legitimately worried about them. The Democrats, sensing opportunity, also are investing.

Congressional Leadership Fund, a super PAC affiliated with Speaker Paul Ryan, is spending $2 million in television and digital advertising to protect Denham in the northern Central Valley-area 10th district.

American Action Network, a political nonprofit associated with Congressional Leadership Fund, is active for Valadao in the southern Central Valley-area 21st district. The group is spending $750,000 for television and digital advertising.

The National Republican Congressional Committee, the House GOP's campaign arm, has spent more than $1.3 million on advertising to defend Knight in the Southern California-area 25th district. American Action Network is spending $325,000 on voter turnout.

Issa hasn't received any help in his San Diego-area 49th district.

But that's because the congressman had a sizable war chest, and as perhaps the wealthiest member of Congress can afford to write his campaign a check if he needs more money.

California operates a top-two primary system, meaning the top two vote-getters in the primary advance to the general election regardless of party.

Issa won his by only 6 percentage points. Another sign that that he might be endangered is the fact that he's been dropping direct mail pieces highlighting his bipartisan cooperation with President Obama.

That's an about face for Issa, who regularly investigated the Obama administration as chairman of Oversight and Government Reform and was among the president's harshest critics.

"This is now a guy who because Trump's poll numbers are bad has sent of brochures with my picture on them touting his cooperation on issues with me," Obama said Sunday while raising money for Democrats in California.

"That is the definition of chutzpah. Here's a guy who called my administration perhaps the most corrupt in history," Obama added.

Issa responded sharply. "I'm disappointed but not surprised that the president, in a political speech, continues to deny accountability for the serious scandals that happened under his watch where Americans died overseas and veterans have died here at home," he said in a statement reported by Politico.

Nationally, Clinton led Trump by more than 5 percentage points in the RealClearPolitics average of recent polls.

That lead was built partly on Trump's weakness in traditionally Republican strongholds like Arizona, Georgia and even Texas. In blue states like California, where the electorate is liberal and ethnically diverse, that underperformance is more pronounced.

Voters there were naturally more likely to reject Trump's strong opposition to illegal immigration and skepticism of legal immigration and foreign trade.

The harsh rhetoric he has used to describe illegal Mexican immigrants, and clunky outreach to other non-white voters, also suggested Trump would have problems in a state like California, which has a high concentration of minorities and moderate coastal Republicans.

And that's exactly what happened, as underscored by the danger Issa, Knight, Denham and Valadao find themselves in.

Issa's district leans Republican. But it's nearly 26 percent Hispanic, and split the last two presidential elections, voting narrowly by Obama in 2008 and by six points for Republican Mitt Romney in 2012.

Denham's 10th district is practically a swing district, with a Demographic makeup that is favorable to Democrats in presidential elections. The seat is 40 percent Hispanic and 6 percent Asian, and voted for Obama in both of his elections, elections, by 2 and 3 points, respectively.

Valadao's 10th district leans Democratic, is more than 70 percent Hispanic, and twice voted solidly for Obama.

Knight's 25th district leans Republican. But it is nearly 40 percent Hispanic and includes a population that is 8 percent Asian and 7 percent African American, and delivered close victories for Obama and Romney.

Add Trump's unusually low numbers to the mix, and down ballot Republicans' challenge increases exponentially. "Here's the problem for California House Republicans. You have a presidential nominee who's not turning out the base vote," Whalen said.

Another impact on turnout is the fact that Republicans don't have a Senate candidate to vote for, because none of the Republican candidates emerged from the primary under the state's top-two primary system.

Democratic Sen. Barbara Boxer is retiring, but both candidates on the ballot are Democrats.

Disclosure: The wife of the author works as an adviser to the NRCC.