And others may soon adopt that strategy, including billionaire activist Tom Steyer, who said Sunday he is considering a run. De León announced his Senate run in a video Sunday.
Their ability to even broach the prospect without immediate political blowback demonstrates how the political landscape has changed since Feinstein was first elected in 1992. Democratic Party tensions have been heightened by last year's bitter primary battle between Hillary Clinton and Bernie Sanders, and took on greater importance after Trump took office in what state Democrats have branded as the liberal resistance to his agenda.
Feinstein "is from the old Senate school of 'Can't we all get along with each other reaching across the aisle?'" said grass-roots environmental activist RL Miller. One bit of evidence for De León supporters like Miller: Feinstein supported half of Trump's Cabinet appointees, substantially more than newly elected California Sen. Kamala Harris. And Miller believes activists are excited by the idea of "resistance."
Any effort by De León or Steyer would have an ironic precedent in the strategy used by the tea party to topple entrenched Republican politicians by stoking antipathy toward a sitting president and the establishment. It's an approach that worked against GOP lawmakers including House Majority Leader Eric Cantor in Virginia in 2014 and veteran Sen. Bob Bennett in Utah in 2010.
Such an effort against a Democrat as prominent as Feinstein, a revered party elder who has served in the Senate for a quarter-century, would be a surprise. However, California may be the sweet spot for this sort of message, given Trump's deep unpopularity here, the vocal Sanders' supporters who already have upended the state party, as well as Feinstein's record as a negotiator open to bipartisan compromise rather than as a liberal flame-thrower.
"She represents a brand of politics that is much more effective in the Capitol than it is on the campaign trail," said Dan Schnur, a political communications professor at USC. "The base of the Democratic Party isn't just increasingly progressive. They're increasingly angry."
Qualities associated with Feinstein — "patience, bridge-building and compromise" are "not a message that plays very well at rallies and protests," he said, adding that she remains a formidable force.
Feinstein, 84, has never been a strident partisan. She used a 1990 speech at the California Democratic Party to express her support for the death penalty, a moment her campaign knew would draw loud boos. It was filmed it and turned into a television ad to show Californians her moderate tack.
She prides herself on her ability to use her seniority and her relationships across the Senate as the most powerful ways to help her home state, regardless of whether Democrats or Republicans control the chamber or the White House.
"It's my way of working to make whatever we do meaningful," she told donors at a fundraiser in Beverly Hills on Tuesday. "You can stand up in this arena … and pound your chest and you don't get anything done. You need to work with people."
That lack of chest pounding, compounded by her call in August to have patience with Trump's presidency, is precisely what enrages her critics.
Both De León and Steyer have made Feinstein's comments about Trump a rallying cry as they have weighed challenging her. The senator is in a difficult position because she sits on Senate committees that are investigating Russian meddling in the 2016 election and the Trump campaign's dealings with Russia. Those closest to her say she would never jeopardize those investigations regardless of the political cost.
Her recalcitrance to making bold statements, such as calling for Trump's impeachment, provides an opening to rivals who see how Sanders' popularity among the state's most liberal voters has manifested itself in ugly protests and a bitter chairperson's election at the state party convention earlier this year.
The question is whether this movement is strong or just vocal.
"There's clearly energy and angst out there. It's just unclear if that energy and angst will be directed in a way you could knock out someone like Dianne Feinstein," said Andrew Acosta, a Democratic strategist based in Sacramento who is not aligned with either candidate.
He noted that the issues captivating the party's progressive flank, such as single-payer healthcare, offer opportunities for candidates on Feinstein's left. But, he cautioned, "that gives you an opening, but I don't know if that's how you finish the game. It just gets you into the game."
Another potential hurdle is how much appeal De León or Steyer will have with these voters. Both have progressive bonafides. De León has championed causes such as immigrant rights and clean energy. Steyer has used his personal wealth — spending at least $91 million in the 2016 election cycle alone — to elect liberal candidates and push liberal policies, notably action on climate change.
Their backgrounds may create problems if they try to paint themselves as the heirs to insurgent movement sparked by Sanders. Steyer is a billionaire former hedge fund manager. De León, a political insider who has served in the Legislature since 2006, regularly raised money from corporate interests with business before lawmakers. He became the first Latino Senate pro tem in more than a century in 2014.
When it comes to fundraising, De León lacks the donor base of other candidates who have run for statewide office. The nearly $3 million he has in state campaign accounts can't be used in a federal race. He does enjoy support from donors because of his leadership role, but they may be reluctant to open their wallets and anger Feinstein.
"There are very few political contributors that are going to want to bet against a 25-year incumbent senator sitting in Washington casting votes all through next year," said Darry Sragow, a Democratic strategist who managed Feinstein's unsuccessful run for governor in 1990.
Steyer can easily self-fund his campaign but runs the risks of other wealthy candidates who have never held elected office. Arnold Schwarzenegger, who had universal name identification, is the lone exception among a bipartisan field of unsuccessful statewide hopefuls.
Some Feinstein-sympathetic Democrats argue that the end result of a primary challenge would be an expensive intraparty brawl that harms their ability to flip GOP congressional seats in districts Clinton won in 2016 — a keystone in national Democrats' hopes to retake the House.
"That's why a lot of people think it's a damn shame," Democratic strategist Steven Maviglio said.
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5:20 p.m.: This article was updated with De León's announcement.