Orange County’s independent voters heavily favor a Democrat Congress member in the county’s four Republican-held congressional districts, underlining the GOP’s vulnerability in these longtime strongholds, according to a poll released Friday.

Of independent and third-party voters in the county’s four GOP congressional districts, 46 percent favor a Democratic candidate in next year’s election and 21 percent prefer a Republican.

Overall, voters in those districts prefer a Republican, 44 percent to 41 percent. But unaffiliated voters’ strong preference for a Democrat could spell trouble, especially since all four districts voted for Democrat Hillary Clinton last year and two key policies of President Donald Trump are viewed unfavorably by voters there.

While the survey was done by a Democratic pollster, non-partisan political data expert Paul Mitchell said the methodology, questions and results showed no signs of bias. Mitchell’s company, Political Data Inc., is the state’s leading provider of voter information and electoral analysis to both Democrats and Republicans.

“It would be really distressing for me, if I was a Republican, to see strong support only in the Republican base — that it didn’t spill into the moderates,” said Jonathan Brown of Claremont-based Sextant Strategies & Research, which conducted the survey. He said the company was not paid for the poll. Rather, it chose to so the survey so it could analyze the difference between phone and internet results, with both methods being used.

The poll provides fresh insight into the vulnerabilities of the Republican incumbents, who have already been identified by numerous experts as being susceptible to strong challengers next year.

The four are among 23 targeted nationwide by the Democratic Congressional Campaign Committee, which has already paid for two Orange County-based organizers and has announced plans to move its Western regional office from Washington, D.C., to Irvine.

Additionally, the Cook Political Report recently changed three of the districts from “likely Republican” to “leans Republican” for next year’s races. Those changes were for Reps. Mimi Walters, R-Laguna Beach; Dana Rohrabacher, R-Costa Mesa; and Ed Royce, R-Fullerton. The bid of Rep. Darrell Issa, a Vista Republican whose district straddles the Orange-San Diego county line, had previously been declared a “toss up” after winning by just 0.6 percentage points last year.

The 10 Democratic challengers to emerge in the four districts so far have all worked to link the GOP incumbents to Trump, whose positions on healthcare and the environment are not popular. In the Republican districts, 47 percent opposed the GOP healthcare bill and 45 percent favored it. When the question was modified to add that it was “strongly supported by President Trump,” 50 percent opposed it and 38 percent supported it.

All four Orange County Republicans voted for the healthcare bill, which now awaits Senate deliberation.

When it comes to the environment, 51 percent of those in GOP districts disapproved of Trump’s positions and 37 percent approved, according to the poll.

Dave Gilliard, campaign consultant for Issa, Walters and Royce, pointed out that the poll asked voters about generic Republicans and Democrats rather than specific individuals.

“In Orange County, we don’t have generic Republicans,” he said. “We have strong incumbents like Ed Royce and Mimi Walters who have a history of attracting voters from across the aisle.”

Surfside resident Shawn Steel, former chairman of the California Republican Party, was dismissive of the poll, pointing out that most pollsters got last year’s presidential election wrong and that a poll by a Democrat was particularly suspect. But when asked about Paul Mitchell, who said the polls was unbiased, Steel called him “the guru.” And Steel readily acknowledged the fight Republican Congress members will have next year.

“In terms of Orange County, it’s a battleground. No doubt about it,” Steel said. “Any Republican who wants to win is going to have to work hard.”

The lack of Republican support from independent voters is significant because those voters typically split the same way as partisan voters and the four contested districts all have more Republicans than Democrats. The polls also found that those on both sides are more engaged than usual with what’s going on in Washington, but particularly Democrats.

“The increase in activism is real,” Brown said. “Whether that leads to increased turnout remains to be seen.”

The poll consisted of 400 phone interviews and 333 internet surveys, with a margin of error of 3.55-percentage points. When adjusted for demographics — internet users are more likely to be young and Democratic than those with landlines — the results from both phone and internet participants was similar, Brown said.

The survey also broke down results by congressional districts. However, those sample sizes were much smaller, did not include portions of the district outside the county and so were subject to larger margins of error.