There's about to be a Democratic civil war in California
Kamala Harris, the junior senator serving alongside Feinstein in California, immediately backed her reelection bid because "Dianne is someone who sticks to her principles and achieves results regardless of powerful opponents, from the assaults weapons ban to the CIA torture report."
But others in the Sunshine State ramped up the flurry of criticisms that have been lobbed at Feinstein in recent months.
Rep. Ro Khanna, a freshman congressman in California, called Feinstein "out of touch with the grassroots" on both economic and foreign policy, according to Vox.
"The fact that the establishment is rallying around her re-election shows that DC insiders continue to privilege protecting one of their own over the voters' concerns," Khanna said. Vox also noted that Khanna pointed out he would not be seeking Feinstein's seat.
The Justice Democrats, a group that pushes to oust establishment candidates in the party, also railed against Feinstein. A spokesperson told Vox's Jeff Stein, "We think she'd be more at home running in the Republican primary."
Joseph Sanberg, a wealthy entrepreneur in California who has been rumored to be mulling a primary challenge, wrote on Twitter shortly after Feinstein's announcement, "Here at home, Californians are leading the resistance to bullies like #Trump. But we need representatives willing to do the same in DC."
"California deserves a bold progressive fighter who will stand up to Trump - bullies like Trump are defeated by courage, not patience," Sanberg added.
Markos Moulitsas, founder of the left-wing website The Daily Kos, encouraged California State Senate President Pro Tempore Kevin de Leόn to challenge Feinstein.
"We share a common interest in this Senate race," Moulitsas wrote. "Let's beat the most pro-Trump Blue-state Dem in the country!"
Unlike some Democrats' calls for impeachment and full-on resistance against President Donald Trump, Feinstein has so far stayed her hand during the new presidency.
During the response to the multiple hurricanes that plagued the American South, Feinstein said at a town hall that Trump "has the ability to learn and to change and if he does he can be a good president."
"This is his first big American emergency," she said. "I think we have to have some patience. I do."
Feinstein later clarified that she still opposes the president on a policy level.
"The duty of the American president is to bring people together, not cater to one segment of a political base; to solve problems, not campaign constantly," Feinstein said. "While I'm under no illusion that it's likely to happen and will continue to oppose his policies, I want President Trump to change for the good of the country."
Feinstein has also made waves in her own party for saying in September that President Barack Obama's Deferred Action for Childhood Arrivals program was "on shaky legal ground." Feinstein said that while she supports DACA, it should be solved through Congress and not the executive branch.
Feinstein drew more criticisms this weekend when she suggested on CBS' "Face the Nation" that no laws would have prevented the Las Vegas shooter from carrying out his massacre.
"He passed background checks registering for handguns and other weapons on multiple occasions," she said.
While many of the top Democrats in the progressive wing of the party have yet to weigh in on supporting her reelection, Feinstein is facing mounting criticisms going into 2018.