Kevin de León. (Jae C. Hong/AP)

Manuel Pastor is a sociology professor at the University of Southern California and the author of “State of Resistance: What California's Dizzying Descent and Remarkable Resurgence Mean for America's Future.”

For many out-of-state observers, it was a startling twist. On July 14, Dianne Feinstein, a Democratic senator from California for more than a quarter century, failed to capture the endorsement of her own state party executive committee. Instead, the nod went to Kevin de León, a termed-out state senator who had scored 12 percent — more than 30 points behind Feinstein — in the state's top-two primary. (A Golden State oddity permits two members of the same party to run against each other in November.)

The impact on the fall elections is debatable. De León will receive party dollars and be listed as the officially endorsed Democratic candidate on "slate mailers," which might tilt the few voters who do not immediately toss those voting guides in the trash. But his name recognition remains low while Feinstein has both loads of campaign cash and a list of big-name endorsers, including former president Barack Obama and Gov. Jerry Brown.

Before the 85-year-old incumbent senator dismisses this episode as an inconvenient speed bump on her inevitable victory lap, she might want to recall Joseph Crowley, the veteran congressman from New York who was toppled by Alexandria Ocasio-Cortez a few weeks ago. Running sharply to his left, the Bronx-born Ocasio-Cortez tapped into a wave of grass-roots anger that wants to replace the traditional politics of accommodation with a more fiery strategy of confrontation.

Are we witnessing the peculiarities of two states that tend to prompt eye-rolling elsewhere in the country? Or is this part of the same zeitgeist that led Bernie Sanders and Donald Trump to catch fire in 2016 — not by moving themselves to the middle but rather by moving the middle to them?

De León has been counting on exactly that polarizing energy since he began his race. Frustration with Feinstein was building through 2017, with some calling for her to step aside and make way for a new generation of leadership. She didn't, and de León promptly followed her October 2017 reelection campaign announcement with a declaration of his own candidacy.

The tension between progressives and pragmatists in California — and between de León stirring up the activist base and Feinstein massaging the supposed center— is not new. De León actually started his political career by helping to organize a large protest against Proposition 187, the 1994 ballot measure that unsuccessfully sought to strip undocumented immigrants of access to nearly all social and educational services. Feinstein eventually opposed the measure, but only after running ads that billed her as having "led the fight to stop illegal immigration."

Feinstein has tried to swing left in step with the state's voters; while it has proved difficult to live down her conditional statement that Trump could be a "good president ," she has pointed to her long support of gun control and showed increasing determination to protect immigrants and turn back judicial nominees.

But de León has the chops that make him a favorite of younger progressives. In his role as a state senator (including his time as that body's leader), he pushed bills to address climate change, raise the minimum wage and expand health care. One of his most prominent efforts: legislation sharply limiting cooperation with Immigration and Customs Enforcement, a "sanctuary state" policy made real last year as the California Values Act.

He also seems to recognize that he is just part of a broader arc of change in California — one powered by the grass-roots groups that often get short shrift from pundits and political consultants.

After all, the state's journey from Proposition 187 to "sanctuary state" required legislative action, but it was pushed every step of the way by immigrant advocates. California's current budget surplus may be claimed by the tightfisted Brown, but the "millionaires tax" that raised needed revenue was forced on him by activists threatening to run their own ballot measure. Even the state's celebrated decision to raise the minimum wage to $15 by 2022 was an attempt to stave off an even more aggressive schedule offered by labor and community allies.

Taking his cue from such determined activism, de León has knocked on doors, shown up everywhere and excited constituencies looking for political daring. Feinstein, meanwhile, has counted on traditional sources of power: money, endorsements and the benefits of incumbency.

His effort may not be enough to win a Senate seat in 2018, but it is clearly part of a nationwide battle for the soul of the Democratic Party. California — whose history of anti-immigrant fervor and racial turmoil has echoes in contemporary America and whose current demographic diversity foreshadows the nation's future — may be one place to get an early glimpse of how that fight will play out.