For California Rep. Dana Rohrabacher, the 2018 election might be his toughest bid yet.

The longtime Republican congressman narrowly trailed Democratic challenger Harley Rouda in a Monmouth University poll of registered voters in California’s 48th District released Tuesday.

Rohrabacher was behind Rouda in all three of the poll’s likely voter models. In the full sample of potential voters — registered voters in the district who have cast ballots at least once since 2010 or are newly registered — Rouda led the incumbent 46 percent to 43 percent, though that was within the survey’s margin of error.

Rouda had a strong lead among independents, a critical swing constituency in a district with more registered Republicans, with 49 percent saying they favored the challenger and 31 percent favoring Rohrabacher. 

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First elected in 1988, Rohrabacher has coasted to victory in his previous elections, winning all but one of them by double digits. He finished first in the top-two primary last month with 30 percent. Rouda was in second with 17 percent. 

But November could see a shakeup in the district if Rouda can successfully turn out voters in the traditionally Republican district that Hillary Clinton carried by 2 points in 2016.

“Rohrabacher came into this race with a target on his back that included some opposition within his own party,” said Patrick Murray, the Monmouth poll’s director. “He retains strong support in GOP strongholds centered around Newport Beach, but voters in the rest of the district are willing to look at someone new.”

Racial and education demographics could also play a key role in deciding the outcome in November.

Rohrabacher led by 21 points, 55 percent to 34 percent, among white voters without a college degree. College-educated whites were essentially split between the two candidates.

Rouda held a 27-point advantage, 57 percent to 30 percent, over the incumbent among black, Latino and Asian-American voters, who make up roughly a third of the district’s registered voters.

“Minority voters tend to show less interest in the election at this point, so a key for Rouda is motivating this part of his base to turn out,” Murray said.

Rohrabacher has been criticized for his support of Russia and its president Vladimir Putin and was quick to come to Trump’s defense after his much-maligned news conference with the Russian president in Helsinki on Monday.

Trump drew a moral equivalence between the U.S. and Russia when he said he held “both countries responsible” for their strained relations.

“I think that the United States has been foolish. I think we’ve all been foolish. … We’re all to blame,” the U.S. president said.

But Trump only said what Rohrabacher has been saying for years now: Russia gets an unfair shake from American lawmakers who have blundered interventionist efforts for decades across Central America, Asia and the Middle East, and most recently in the Ukraine, Iraq, Libya and Syria.

“They found it in their interest to meddle in our elections,” Rohrabacher said of Russia in a heated interview Monday on Bloomberg Television. “But what the point is — we meddle in their elections. We meddle all over the world at a much higher rate than what Moscow does. And maybe it’s all wrong. But the fact is we helped overthrow the government of Ukraine.”

The Monmouth poll surveyed 402 voters in the district by telephone from July 11 to 15. It had a margin of error of 4.9 percentage points for the full sample and 5.2 points for the likely voter model.