Trump, who at least temporarily has shuttered his California campaign offices, trails Clinton by 30 points in a two-person race and by 24 points when Libertarian Party candidate Gary Johnson is thrown into the mix.
Moreover, Trump is faring "historically" poorly with independent voters, Democrats and Latinos, Field Poll Director Mark DiCamillo said.
While the survey of 956 likely California voters was completed three days before FBI Director James Comey took Clinton to the woodshed over her "extremely careless" email use while secretary of state, it demonstrates the huge obstacles Trump faces in the Golden State and the demographic difficulties that could plague him in several swing states with large Latino populations.
"If Clinton had been indicted, things might be different, but as it stands now, Trump has virtually no chance in this state," said Jack Pitney, a professor of politics at Claremont McKenna College.
And if his polling doesn't pick up, Trump could make it challenging for several GOP congressmen in California to keep their seats, said Bill Whalen, a former Republican operative, who is now a research fellow at Stanford's Hoover Institution.
"The biggest winner in this poll might not be Hillary Clinton. It might be Nancy Pelosi," he said, speaking of the House minority leader, who wants to once again be speaker.
Trump insisted he could win California while stumping in San Jose in May, and his state political director Tim Clark said the race would tighten once Trump starts up his ground game out of local GOP offices.
"We haven't rolled out our grass-roots army yet for the fall," he said. "I think we will have a very united Republican Party in a state that always favors the independent-minded candidate and the candidate who is the job creator."
But it's hard to fathom Trump prevailing when the poll shows him trailing Clinton among California Latinos 75 percent to 12 percent in a two-person race. With Johnson included, Trump's share of the Latino vote sank to just 9 percent.
That's a long slide from the days of Ronald Reagan, who won 40 percent of Latinos in 1980.
"When you are getting single-digit support from a constituency that is 25 percent of registered voters and increasing, that is almost insurmountable," DiCamillo said.
If those numbers hold among Latinos around the country, Pitney said it would doom GOP chances in traditional battleground states such as Florida, New Mexico, Nevada and Colorado.
"For the Republican Party, Donald Trump is one big orange anchor," he said.
Clinton led Trump 58 percent to 28 percent in a two-person race, the poll found. With Johnson included, Clinton led with 50 percent, followed by Trump at 26 percent and Johnson at 14 percent.
Her wide margin appeared to be a product more of Trump's weakness than her strength, DiCamillo said. Nearly one-third of voters with no party affiliation said they were still undecided. Of the rest 48 percent chose Clinton and 20 percent said they planned to vote for Trump.
Clinton led among every age group, income level, education level and region, including the Central Valley. She had the backing of 16 percent of Republicans, while only 5 percent of Democrats said they planned to vote for Trump.
While it's still too early to determine how much Clinton will be damaged from Tuesday's stinging rebuke over her handling of classified information in emails while secretary of state, several independent voters said Comey's remarks hadn't changed their minds or made them consider voting Republican.
"It's been beaten to death," said Michael Kogut of Los Altos, who still plans to vote for Clinton. "I think it's something that a lot of people could have done."
But Robert Potmesil, a 47-year-old Milpitas resident, said the FBI director's critique of Clinton reinforced his own mistrust of her and helped reaffirm his intention to vote for Johnson in November. "I prefer Trump to Hillary," he said. "That's how horrible I think Hillary is."
However, George Miller, a 33-year-old Hayward resident, who supported Sanders in the Democratic primary, said he was more likely to vote for Clinton now that it's certain she won't be indicted.
"I can't say that I've made that decision yet," he said, "but I'm more comfortable with the idea of voting for her now."
By Matthew Artz, The Mercury News