De León has begun calling labor leaders and elected officials to inform them of his plans, the sources said, and is expected to soon announce his campaign against Feinstein, a giant of California Democratic politics who has held the office since 1992.
The 50-year-old de León, who represents Los Angeles and is seen as a leading Latino voice in Democratic politics, is likely to campaign aggressively against President Donald Trump. He began signaling he could oppose Feinstein in late August, after she said Trump could "be a good president" and that he "can learn and change." Feinstein later clarified that she is "under no illusion that it's likely to happen and will continue to oppose his policies."
He will enter the race after a state legislative session in which California lawmakers pushed legislation targeted at Trump -- making California a "sanctuary state," sought to force Trump to release his tax returns by making it a requirement to appear on the state's ballots as a presidential candidate, and passed a resolution urging Trump "to publicly apologize to all Americans for his racist and bigoted behavior."
"California's progressive and labor movement is salivating for new blood and fresh ideas in Washington, and Kevin de León is the embodiment of the California resistance and the future of the Democratic Party," said California Democratic strategist Dave Jacobson.
De León likely won't be the only Feinstein challenger. Tom Steyer, the billionaire environmentalist and single largest Democratic donor, is "very much looking at the Senate race," a source close to him said. Steyer, who is in Washington this week, issued a letter Wednesday challenging Democratic candidates to call for Trump's impeachment.
"This is no time for 'patience' -- Donald Trump is not fit for office. It is clear for all to see that there is zero reason to believe 'he can be a good president,'" Steyer said in the first line of his letter.
With criticism of Feinstein mostly coming from the left, Democratic candidates seeking to replace her will first have to win over the party's increasingly powerful progressive wing. Because there is no party primary in California -- the top two overall vote-getters in the June "jungle primary" advance to the general election, no matter their affiliation -- the race will be fought on two planes.
Feinstein figures to win the backing of the party establishment, and will almost surely advance past the June vote to the November election. That means her challengers will effectively be running their own mini-race, against each other, for the chance to take her on head-to-head.
The roster of high-profile contenders is expected to expand beyond de León. The Justice Democrats, an increasingly influential progressive grassroots group, does not plan to back him
"We are going to launch another candidate, next week -- someone that's not announced at all yet," said Corbin Trent, the Justice Democrats communications director.
"It's going to be her," Trent said -- not naming names. "We're certainly not going to try to whittle away at the already shameful 17% representation women have in the the House and Senate."
There is also the prospect of Joe Sanberg, the wealthy investor and activist, who is now seriously considering a run. He paid for and launched a digital media buy last month, along with a website to introduce himself, and his economic plan, to voters. Sanberg's decision could be influenced by de León's early performance. But Sanberg adviser Rebecca Katz, a veteran progressive operative, speaking Thursday took aim at the incumbent -- and the President.
"Dianne Feinstein is one of the most conservative senators in the Democratic caucus," she said. "And in the age of Trump a state as progressive as California deserves the most progressive senators."
De León will enter the contest with a significant recent boost to his progressive bona fides. As state Senate president, he passed a bill out of his chamber that would lay the groundwork for a single payer health care plan in California. That legislation, SB562, was subsequently tabled by state assembly speaker Anthony Rendon, setting off a fierce round of intra-party recriminations.
"Our relationship with de León has been extremely positive. He's taken a lot of heat (for moving the single payer bill), as the cowardly Democrats in the assembly try to hide," said RoseAnn DeMoro, executive director of National Nurses United, which lobbied hard for the bill. "De León stood up."
The nurses' union has been approached by potential candidates but has not yet not made a decision on who it might endorse in the race. DeMoro, a fiery critic of Democratic establishment figures and staunch supporter and ally of Sen. Bernie Sanders, expressed her personal respect for Feinstein, but is concerned the senator's re-election bid is a "ploy" to clear the way for a more moderate future successor, like Democratic Rep. Adam Schiff.
"I hate that she's put us in this position because I don't want to disrespect her history and that's the problem that we've got here," DeMoro said. "It's not her powerful position, and she is powerful, but we need someone who reflects today's values as a leader. It's not the age thing, it's the political thing. I don't think she's in this for the long haul."
As for Sanders, there is no indication as yet that he is planning to engage in the race. He has mostly stayed clear of the Democratic Party's most pitched battles, instead focusing on his "Medicare for all" push and support for progressive candidates in local races.
He endorsed Randall Woodfin, who was elected mayor of Birmingham, Alabama -- defeating an incumbent Democrat -- earlier this month. And the political organization spun out of his 2016 campaign, Our Revolution, is finding success down the ballot too, with a series of recent endorsements, from Birmingham to New York, where Christine Pellegrino, a Democrat, shocked observers last spring by defeating a Republican in a state assembly race.
CORRECTION: This story and headline have been updated to correctly describe California's primary system, which is not party-specific.