The Democratic leader of California’s state Senate has announced he will challenge incumbent Democratic Sen. Dianne Feinstein, setting up a generational clash over liberal California.

From the moment Donald Trump won the presidency, elected officials in deep-blue California have vowed to block his agenda and preserve California’s robust environmental laws and protections for immigrants.

Senate President Pro Tem Kevin de León has been leading the pushback, vociferously condemning Mr Trump and accusing him of racism, and he has been increasingly critical of Sen Feinstein’s more conciliatory stance toward Mr Trump.

After Sen Feinstein called for “more patience” with the president, Sen de León lambasted her for being “complicit in his reckless behavior”. He broke with the senator last week after she said no law could have stopped a mass shooting in Las Vegas, arguing that tougher safeguards would make a difference.

Earlier this week the 84-year-old Sen Feinstein dispelled rumors she would be stepping away from her decadeslong Senate tenure, confirming she would run for reelection. While other ambitious California politicians lined up behind her, some argued it was time for a changing of the guard.

Now, with de León’s entrance, California will host what could become a costly and bitter fight over the future of the Democratic Party in the era of Trump. Both candidates have deep reservoirs of political and financial support from their years in politics.

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It also suggests that Democrats, whose base is energized by a deep antipathy to Donald Trump, are not immune to the type of primary challenges that in recent years have tended to target mainstream Republicans. Grassroots activists in California and beyond have pressured their Democratic elected officials to more aggressively confront Mr Trump or face a reckoning at the polls.

The state’s primary system allows the top two vote-getters in the primary to advance to a general election runoff regardless of party. That means voters could have a choice next November between two Democrats in Sen Feinstein and Sen de León.

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While Sen Feinstein is well known to California voters thanks to a long career in San Francisco politics and then the US Senate, de León will be able to rally some of the state’s huge Latino electorate and can point to his more recent efforts to defy a president who remains deeply unpopular among the Democratic base.

He was a driving force behind a so-called “sanctuary state” law, recently signed by Gov Jerry Brown, that imposes sweeping limits on local police and sheriff’s officials working with federal immigration authorities, and he has pursued aggressive new renewable energy targets to help California combat climate change.