Biden, she said, “will continue the fight to restore the soul of the nation from the Oval Office.”

While Harris rolled out her White House run before a crowd of 22,000 supporters earlier this year and has managed to land a slew of big-name California endorsements — including from Gov. Gavin Newsom — her failure to snag the endorsement of Feinstein, who's been a fixture in the state's politics for generations, is seen by many as another blow to her 2020 campaign, which has lagged in recent polls.

Still, in California, where many have long viewed the two as having an uneasy relationship, few were surprised — especially because Feinstein and her husband, Richard Blum, hosted Biden at a fundraiser in their home last week.

“Dianne has known Joe longer than Kamala’s been alive,’’ one high-profile Democratic strategist who knows both said, speaking on the condition of anonymity.

Harris’ team, including her longtime adviser Debbie Mesloh, downplayed the differences and said the two share much common ground, having “worked together over the years on issues that are important to Californians.”

“Kamala served as D.A. of Sen. Feinstein’s hometown of San Francisco and was supported by the senator when she ran and won to be California’s first female attorney general,’’ Mesloh told POLITICO. “In the Senate, they have worked together on the Senate Intelligence Committee.”

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Mesloh said Feinstein, as one of the longest-serving members of the Senate, naturally has many long-term relationships in that body, but “what will be important is that we are all united after the primary to beat Donald Trump.”

Others said Feinstein’s endorsement is the result of a longtime political relationship — and a pragmatic view of the 2020 election.

“Feinstein endorsing Biden is mostly about Biden,’’ said Rose Kapolczynski, a veteran Democratic strategist who has advised former Sen. Barbara Boxer and Tom Steyer, a billionaire activist who is also running for the Democratic presidential nomination. “They served together for 15 years in the Senate, which is a very small, collegial body.’’

Feinstein and Biden, who represented Delaware in the Senate, have worked side by side on key issues, and he was at her side in some of her toughest battles.

“As soon as Feinstein got elected to the Senate and was facing a tough reelect, her assault-weapons ban got incorporated into Biden’s crime bill — and they worked together on that,’’ Kapolczynski recalled. “It was one of her signature accomplishments and crucial to her politics.’’

Through the years, Feinstein and Biden — the key link to the Senate of President Barack Obama, who regularly reached to the chamber — developed a political relationship “that is long and deep,’’ she said.

Stylistically, too, there’s an affinity, Kapolczynski said: “Both Dianne and Joe are pragmatists. They want to hammer out the details, bring people together and get it done. … So it’s very natural that she would choose Joe out of this field of candidates.’’

Privately, some California Democrats say there has long been a gulf between the 85-year-old Feinstein’s formality and centrist politics and Harris, who at 54 is a generation younger.

Their political differences played out in the headlines early in Harris’ career when, as San Francisco district attorney in 2004, Harris said she would not seek the death penalty for a gang member who fatally shot a 29-year-old police officer, Isaac Espinoza.

Feinstein, addressing the congregation at St. Mary’s Cathedral during Espinoza’s memorial, delivered a veiled rebuke of that decision as Harris sat in the front row. “This is not only the definition of tragedy, it’s the special circumstance called for by the death penalty law,” Feinstein said, as officers filling the pews shouted their agreement.

Kapolczynski recalled the political shock waves of the day.

“It was very unusual for a senator to publicly call on a local D.A. to change her position — and she refused,’’ Kapolczynski said. “You get the impression they never had much of a relationship.”

Still, others suggest the senior senator had hoped her daughter, Katherine Feinstein, who served as a presiding judge on the San Francisco Superior Court, might yearn for public office and someday run for the seat now held by Harris.

In 2016, asked about a possible Harris run, the elder Feinstein didn’t mask her doubts or hide the distance between herself and an ambitious younger senator who had quickly garnered a national spotlight. While saying she was “a big fan” of Harris, Feinstein added, “She’s brand-new here. … It takes a little time to get to know somebody.’’

“The Senate is a place that respects seniority, and that’s built into all the rules and relationships,’’ Kapolczynski said. “And every new senator is counseled to respect those traditions because you need the votes of everyone in that body to get things done.’’

For Feinstein, she said, there was likely some concern that “Harris got elected and immediately started running for president.’’

Wade Randlett, a Silicon Valley supporter and fundraiser for Biden, said that with her endorsement, Feinstein issued her definitive message to Democrats regarding what she believes the party's most important mission in 2020 should be: beating President Donald Trump.

Feinstein is the rare California Democrat, Randlett said, who — after being first elected in 1992, the “Year of the Woman’’ — has remained popular with voters even in more conservative areas of the nation’s most solidly blue state, like the Central Valley.

“She understands how to win in those places,’’ Randlett said. “The moderate, non-college-educated, non-coastal Democrat — she has always been in touch with those voters. And that’s why she understands that Joe is the candidate who will pull those independent voters in the Midwest and the Rust Belt into the Democratic column.’’