The announcement that Feinstein would run was first tweeted Monday by an unverified Twitter account affiliated with Feinstein. A spokesman for Feinstein confirmed its veracity to CNN.
"I am running for reelection to the Senate. Lots more to do: ending gun violence, combating climate change, access to healthcare. I'm all in!" the tweet said.
Feinstein has been in her seat since 1992. At 84, Feinstein is the oldest senator currently serving in the chamber. She is the top Democrat on the Senate judiciary committee.
Her decision to run for reelection comes in the face of grumbling from progressives in California over some of the senator's recent remarks about President Donald Trump. Among them: She said two months ago that she hopes Trump "has the ability to learn and to change, and if he does,he can be a good president."
The mere implication that Trump could be a "good president" drew the ire of state Senate president Kevin de León, whose recent two-day trip to Washington was seen as laying the groundwork for a potential Senate run.
Markos Moulitsas, the founder of the progressive blog DailyKos.com, tweeted
"let's talk" at de León on Monday.
"We share a common interest in this Senate race. Let's beat the most pro-Trump Blue-state Dem in the country!" Moulitsas said.
Joseph Sanberg, a former Wall Street investor and entrepreneur who successfully lobbied for California's earned income tax credit program, is another potential Feinstein primary challenger.
In a series of tweets
posted after the announcement on Monday, Sanberg sought to make the case that Feinstein had been too deferential to Trump, and that with deep-blue California " leading the resistance" on the state level, "we need representatives willing to do the same in D.C." He did not, however, make clear his own plans.
Feinstein's old-school political sensibilities and foreign policy could be at odds with California, where increasingly young and diverse voters have demanded all-out opposition to Trump. The progressive group CREDO recently confronted Feinstein in her office
to urge her to block Trump's judicial nominees.
Feinstein's California colleague, Sen. Kamala Harris and a rising star among liberals, quickly endorsed Feinstein in a Facebook post.
"I strongly support Dianne's reelection campaign, and am thankful she is again offering to serve our state," Harris wrote
. "What Californians get from Dianne is someone who sticks to her principles and achieves results regardless of powerful opponents, from the assault weapons ban to the CIA torture report. We are better off with her leadership and I look forward to continuing to fight together for California in the Senate."
Los Angeles Mayor Eric Garcetti, meanwhile, plans to host a fundraiser
for Feinstein's re-election campaign, the Los Angeles Times reported last week