Progressives frustrated with Sen. Dianne Feinstein Dianne Emiel FeinsteinMcConnell says Trump nominee to replace Ginsburg will get Senate vote Top Democrats call for DOJ watchdog to probe Barr over possible 2020 election influence Intensifying natural disasters do little to move needle on climate efforts MORE (D-Calif.) plan to keep up the heat on the longtime senator after her Monday announcement that she’ll run for reelection.

After months of remaining silent on a reelection run in 2018, Feinstein tweeted Monday morning that she’s “all in” and wants to continue working on issues including “ending gun violence, combating climate change, access to health care.”

The latest reelection bid from Feinstein, the oldest senator at 84, quickly earned endorsements from leading California Democrats. House Minority Leader Nancy Pelosi told the Washington Post that Feinstein’s retirement would have been a “tremendous loss,” while Sen. Kamala Harris Kamala HarrisHarris honors Ginsburg, visits Supreme Court The Hill's Campaign Report: Trump and Biden vie for Minnesota | Early voting begins in four states | Blue state GOP governors back Susan Collins Kamala Harris: Black Americans have been 'disproportionately harmed' by Trump MORE said California is “better off with her leadership.”

But Feinstein’s announcement was also met with criticism from the left. Rep. Ro Khanna (D-Calif.), who represents Silicon Valley, blasted Feinstein in a statement as “out of touch with the grassroots.”

ADVERTISEMENT

Progressives are wasting no time pushing back on Feinstein, with some are already making public overtures to potential Democratic challengers who could launch primary challenges.

Feinstein, the top Democrat on the Senate Judiciary Committee, has irked progressives who believe she hasn’t done enough to block President Trump Donald John TrumpUS reimposes UN sanctions on Iran amid increasing tensions Jeff Flake: Republicans 'should hold the same position' on SCOTUS vacancy as 2016 Trump supporters chant 'Fill that seat' at North Carolina rally MORE’s nominees.

And Feinstein has sometimes been at odds with her state party, as Democrats in the Golden State continue to veer to the left. She opposes California’s push for single-payer health care, which was a core tenet of Sen. Bernie Sanders Bernie SandersNYT editorial board remembers Ginsburg: She 'will forever have two legacies' Two GOP governors urge Republicans to hold off on Supreme Court nominee Sanders knocks McConnell: He's going against Ginsburg's 'dying wishes' MORE’s (I-Vt.) presidential campaign in 2016. She also came under heavy scrutiny and raised eyebrows after saying in late August that Trump could “be a good president” and called for “patience” when it comes to his presidency.

“Dianne Feinstein since the beginning has never been the darling of the liberal left, not even as mayor of San Francisco,” said Garry South, a California Democratic strategist. “The Democratic base in California has moved pretty far to the left, even in the last 10 years.”

“You would certainly have to take that into account with respect to the general election runoff between her and another Democrat who would be from the progressive side,” South continued.

Feinstein appeared to brush off a potential challenge from a progressive candidate in an interview with The Los Angeles Times on Monday.

“I am what I am, I’m pretty well known and people, I assume, will come after me any way they can. That’s up to them,” Feinstein said.

California features a “jungle primary” system, where all candidates face off in a single primary, regardless of party. The top two finishers advance to a general election runoff.

Feinstein is still considered to be the heavy favorite, but some progressives and groups were quick to show their willingness to back more liberal challengers.

California Senate Pro Tem President Kevin de León, whose name has circulated for weeks, appears to be at the top of that list. While he has yet to publicly announce if he’s running, the New York Times reported that Feinstein decided to announce her reelection after she heard that de León was preparing to launch a Senate bid soon.

De León’s office did not respond to a request for comment.

De León is the first Latino to serve as the state’s Senate Pro Tem president since the 19th century. He hasn’t been shy about butting heads with Feinstein in the past.

Even with no announcement, some progressives are eager to rally around him, publicly urging him to jump into the race.

De León pushed back after Feinstein’s comments about Trump and sparred with her over gun control, which has been one of her signature issues as an author of the federal assault weapons ban in the early 1990s.

After Feinstein said that no law would have prevented the mass shooting in Las Vegas — though she supports a ban on bump stocks — de León said the U.S. can prevent future mass shootings by “getting weapons designed for the battlefield out of our neighborhoods.”

Following Feinstein’s election announcement, Markos Moulitsas, who is the founder of the liberal blog Daily Kos and based in California, tweeted at de León and asked him to consider defeating “the most pro-Trump Blue-state Dem in the country.”

Moulitsas said he hasn’t spoken to de León before, but believes de León would be “far more in tune with his state” than Feinstein.

“I just wanted to make publicly clear that if there's any doubt in his mind about running, that Daily Kos would be enthusiastically in his corner in the fight,” Moulitsas told The Hill.

“Whether progressives unite against Feinstein remains to be seen, but from my vantage point, there is huge hunger for a viable challenger, and de León fits the bill. If he gets in, this race immediately becomes one of my top priorities.”

Democracy for America (DFA), a major progressive group founded by former Vermont Gov. Howard Dean in 2004, hasn’t made an endorsement but is keeping an eye on de León.

“He is someone we’re watching very, very closely,” DFA spokesman Neil Sroka told The Hill. “We would be especially interested in seeing Kevin run.”

Other potential candidates include billionaire environmental activist Tom Steyer and wealthy entrepreneur Joseph Sanberg. Steyer, who has also considered a 2018 run for governor in California, said he’s still exploring all options.

"The whole political establishment in Washington has failed. As always, I'm committed to moving California forward,” Steyer said in a statement provided to The Hill. “I am still looking at all options and will make an announcement about my intentions very soon."

Political observers in the state say that anyone hoping to take on Feinstein will need to raise tens of millions of dollars. That means the race, in California’s expensive media market, could be more attractive to candidates who can self-fund.

As of late June, Feinstein has nearly $3.6 million cash on hand but will likely report more in the next quarterly reports, thanks to some recent fundraisers.

“The big picture for California is, unless you got the ability to raise $20 million to $30 million dollars as a non-incumbent, forget it,” Bob Mulholland, a California member of the Democratic National Committee (DNC) who supports Feinstein, told The Hill. “We’re not New Hampshire. We’ve got 13 media markets."

It remains to be seen if progressives will unite behind a single candidate to take on Feinstein. Still, many believe there’s an appetite for a primary challenger who could reflect California’s leftward shift.

“There’s no reason why a senator from California isn’t another Elizabeth Warren Elizabeth WarrenBiden's fiscal program: What is the likely market impact? Warren, Schumer introduce plan for next president to cancel ,000 in student debt The Hill's 12:30 Report - Presented by Facebook - Don't expect a government check anytime soon MORE or Bernie Sanders and that’s what we need if we’re going to have not just stand up to Donald Trump, but articulate a progressive vision for the future,” Sroka said.

—Updated at 7:27 p.m.