Sen. Dianne Feinstein could face a stiff series of obstacles as she vies for re-election in California in 2018.

Six years after receiving the most popular votes in any senatorial election in U.S. history, the five-term Democrat has seen sliding favorability ratings as liberals in the Golden State hammer her for not being tough enough standing up to President Donald Trump.

Six months before the 2018 primary, just 41 percent of likely voters in California — a plurality — back the 84-year-old Feinstein, a new poll from the UC Berkeley Institute of Governmental Studies found.

Twenty-seven percent of those polled back Feinstein’s closest challenger so far, Los Angeles-based state senator Kevin de León. A considerable 32 percent said they were undecided, which some experts said could open the door to an insurgent campaign from a political outsider.

“She should be farther ahead,” David McCuan, a political science professor at Sonoma State University in California, told the San Jose Mercury News.

“Someone outside of politics has to be encouraged to at least test the waters,” he said.

Tom Steyer, the heavyweight Democratic donor and environmental activist, has reportedly been weighing an insurgent campaign for months but has not announced an official bid.

Feinstein will have to fend off de León and others in California’s so-called jungle primary, set for June 5. The top two vote-getters in the primary advance to the general election, regardless of party.

Inside Elections with Nathan L. Gonzales rates the general election race Solid Democrat.

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“There are polls and then there are polls. I’m ready for a good fight,” Feinstein said at a campaign event in October. “I’ve got things to fight for. I’m in a position where I can be effective, and hopefully that means something to California.”

De Leon, 50, is taking on his fellow Democrat for not being liberal enough to represent the solid blue state. He has the backing of some key California progressive groups and could collect a number of endorsements from lawmakers such as Rep. Ro Khanna, who has been calling for a Democrat to challenge Feinstein.

Feinstein already has the support of a number of key Democrats, including California junior Sen. Kamala Harris and House Minority Leader Nancy Pelosi.

Feinstein has touted her seniority as an asset, not a detriment, in opposing the Trump presidency, citing her leading role on the Senate Judiciary Committee and activity on the Intelligence and Appropriations Committees.

The Berkeley poll was conducted between Dec. 7 and 16 in both English and Spanish and had an error margin of 3.2 percentage points.