Los Angeles (CNN) Kevin de León announced Sunday morning that he would challenge veteran US Sen. Dianne Feinstein, saying he'll stand for a wing of the party that feels she no longer represents the progressive makeup of the state's Democratic Party and has not aggressively challenged the policies of President Donald Trump.

The bold move by de León, the State Senate president pro tempore who is termed out next year, set up an internecine battle within the Democratic Party that some fear could draw attention and resources away from the seven competitive House races that could flip control of the US House of Representatives to the Democrats.

But de León represents the younger generation of California Democrats who have been frustrated by Feinstein's mild criticism of Trump and the lack of opportunity for higher office because of the lengthy tenure of figures like Feinstein, House Minority Leader Nancy Pelosi, Gov. Jerry Brown and former Sen. Barbara Boxer (who retired and was replaced by Sen. Kamala Harris last year). Feinstein will be 85 at the time of next year's election.

In a video statement released Sunday morning, de León said that in his three years as the State Senate leader he had worked to infuse "progressive California values in important policy efforts like immigration, women's rights, quality education, civil rights, job creation and fighting climate change."

"We now stand at the front lines of a historic struggle for the very soul of America, against a President without one," de León, who is 50, said in his video statement, taking aim at Trump. "Every day, his administration wages war on our people and our progress. He disregards our voices. Demonizes our diversity. Attacks our civil rights, our clean air, our health access and our public safety. We can lead the fight against his administration, but only if we jump into the arena together."

Like Harris, de León has been far more aggressive than Feinstein in his criticisms of Trump, and his political allies say he feels that California needs another senator who could actively challenge Trump in the US Senate.

'Patience'

Feinstein has declined to back calls to impeach Trump and said earlier this year that she believed that Trump could "learn and change" and could "be a good President." When she called for "patience" with his presidency during an appearance at the Commonwealth Club, de León issued a sharp and unusual rebuke of the veteran senator.

"I don't think children who breathe dirty air can afford patience," he said in response to her comments. "The LGBT worker or woman losing their rights by the day or the black student who could be assaulted on the street, they can't afford patience. 'Dreamers' who are unsure of their fate in this country can't afford patience."

Feinstein later clarified her statements about Trump, underscoring that while she wanted Trump to "change for the good of the country" she was "under no illusion that it's likely to happen and will continue to oppose his policies."

In a telephone interview on Sunday, de León demurred when asked to name policy areas where he is more progressive than Feinstein, or specific issues that they disagree on. He said he would discuss those details as the campaign progressed (his formal announcement is Wednesday).

"The state has changed significantly over the last 25 years and we're overdue for a real debate on the issues, priorities and leadership that voters want from their senator," de León said, pinpointing the push for "Medicare for all," climate change and policies affecting DACA recipients.

"We have a President who has working families in his crosshairs and now is not a time to be complacent or accommodating," he said. There are millions of people with no voice. My career has been dedicated to working for them."

As Feinstein weighed whether to run for a sixth term, many of California's top Democrats urged de León not to run. When she announced her re-election campaign on Monday, many top Democrats -- including Harris, Boxer, Lt. Gov. Gavin Newsom, and Los Angeles Mayor Eric Garcetti -- quickly closed ranks around her.

De León's district

De León represents a diverse district that includes much of eastern Los Angeles, including the predominantly Hispanic Boyle Heights, Koreatown, Chinatown, Little Toyko, Thai Town, Silver Lake and Los Feliz. During a recent event on affordable housing, he joked that with Echo Park alone he represented "the largest concentration of hipsters outside of Brooklyn."

He got an early endorsement from Democracy for America, which said California voters "deserve two senators who will be relentless allies in their battle to stand up to the White House and take bold action on social, racial and economic justice."

"When Trump became President, de León didn't ask people to sit back and wait, hoping maybe Trump would someday turn out OK," DFA Executive Director Charles Chamberlain said of the group's decision to endorse de León, who is the first US Senate primary challenger the group has backed. "He immediately began working to pass policies that would help the people most immediately affected by Trump's bigoted, greedy policies."

De León, the son of a single immigrant mother who worked as a housekeeper, championed legislation to make California a sanctuary state, meaning the state will not help federal agents apprehend and deport residents who came here illegally but are working hard and staying out of trouble with the law. He has also been at the forefront on climate change legislation at the California statehouse.

Financial backing

While de León may get considerable backing from California labor and left-leaning Democratic groups, he faces huge hurdles both in fundraising and his lack of name recognition across the state.

Feinstein has the ability to self-finance her campaign, and she is actively raising money for her bid, netting about $100,000 at one recent event in Los Angeles. She had $3.6 million in cash on hand, according to her latest Federal Election Commission report, which covered the period through the end of June.

De León's allies hope he will be able to tap into the national network of donors who want to see the Democratic Party go in a more progressive direction. One wrinkle: de León backed Hillary Clinton in last year's presidential election over Vermont Sen. Bernie Sanders, who drew many of the same liberal voters and millennials who de León will now be courting.

When asked whether he regretted that endorsement on Sunday, de León said "No." He explained that he had a pre-existing relationship with Clinton. He said he did not know Sanders personally when he made his decision, but admired the way he had activated and inspired like-minded voters.

Clinton defeated Sanders in California in the June primary last year with 53% of the vote to his 46%.

Though Feinstein has angered the restive portion of the progressive Democratic base here, few longtime California consultants and pollsters are ready to hazard a guess about just how large that portion of the party is. Feinstein has wide and deep support within the Democratic Party. Her approval rating in the latest Public Policy Institute of California poll stood at 54% among likely voters -- just one point lower than Gov. Brown (55%) and seven points higher than Harris (47%).

Feinstein's allies reject the notion that she has a "base problem." Her longtime adviser Bill Carrick said he has seen no evidence of that in public or private polling, a viewpoint shared by a number of California strategists who are not involved in the race.

Feinstein's popularity

Carrick noted that Feinstein is more popular than any other state official in Northern California, and that she has never lost populous Los Angeles County in any primary or general election race. He also scoffed at the notion that de León has a more progressive record than the senior senator.

"He's a term-limited guy looking for a gig," Carrick said. "She's as strong as she's ever been with Democrats, she doesn't have a problem with the Democratic base. He has a problem with the Democratic base -- they don't know who the hell he is."

But California Democratic Chairman Eric C. Bauman said there was little harm in having a robust debate within the party.

"At a time when Democrats control so little in Washington, the fact that she's a ranking member both on intelligence and judiciary, and is well-respected is an extraordinary benefit to us," Bauman said.

"Beating Dianne Feinstein is a tough hill to climb; she has a lot of advantages. But I think a spirited race," Bauman said, "has one really positive effect. It drives turnout. It excites voters. It makes them feel like something is going on."

Carrick rejected that viewpoint. "I think it is really reckless to get in and challenge a Democratic US senator who has been there over and over again on the important issues for California and Democrats, and not to focus our attention on these seven potential pickup seats in California," he said.

California's primary

Feinstein and de León will also be competing in California's top-two primary system, where all the contenders battle for the seat, regardless of party. That is likely to help Feinstein, because she has demonstrated broad appeal to independents and some moderate Republicans over the years.

Because of the exorbitant cost of television advertising in California's media markets, some political consultants estimate that combative race will cost well more than $50 million and possibly as much as $100 million -- in part because the candidates may have to compete twice, in the primary and general elections.

De León said that was not a reason to shy away from the fight. He said there was no question that his campaign would not be able to match Feinstein dollar for dollar, noting that she's "a billionaire."

"No doubt this will be a challenge, but I've faced challenges my whole life, people telling me that things can't be done," he said. "I know a race like this, going up against a longtime incumbent, won't be easy, but this state needs a different and new kind of leadership."

CORRECTION: This story has been updated to correct Nancy Pelosi's title.