Washington DC - Dianne Feinstein of California, the United States' oldest senator, has passed away.

Veteran Senator Dianne Feinstein, a titan of US political history who notched a string of legislative achievements during a trailblazing three-decade career in the Senate, has died at 90 years old.

Feinstein was celebrated as a pioneer for women in politics and an effective legislator. During a career that began in local California government, she grew to be a tough check on administrations from both parties.

She had already announced she would not run for reelection as her health worsened and following a number of missteps that threatened her legacy.

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The Californian, who was widowed last year, became San Francisco's first female mayor after the fatal shooting in 1978 of Harvey Milk, the country's only openly gay politician, and mayor George Moscone by a disgruntled former colleague.

Other dramatic moments including surviving an attempted bombing of her home. She was also near the scene of an infamous double murder in San Francisco.

Feinstein's death is not expected to shift the tight balance of power in the Senate, with the Democratic governor of California appointing her temporary replacement.

Summing up her dedication to public service, Feinstein once said that "even with a divided Congress, we can still pass bills that will improve lives. Each of us was sent here to solve problems."

"That's what I've done for the last 30 years, and that's what I plan to do for the next two years."

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Democrats in her home state led tributes as news of the senator's death cast a pall over proceedings in Congress, with staff placing flowers on Feinstein's desk in the Senate chamber, according to Fox News.

"She was a trailblazer and dedicated her life to public service," said David Valadao, a member of California's delegation in the House of Representatives.

An icon in Senate politics who took the lead on hundreds of bills, Feinstein rose to the chairmanship of the powerful intelligence committee – another first for a woman.

Her achievements include writing the 1994 assault rifle ban and producing a 6,700-page report on the CIA torture program during the US "war on terror."

But it was in her home state of California where she first made her mark.

As the first female Board of Supervisors president in San Francisco, Feinstein led the city through the tumult that followed the fatal shootings in 1978 of Milk.

She replaced the mayor and served for 10 years as a pragmatist, willing to work across the aisle, balancing nine budgets in a row, and being declared the nation's "Most Effective Mayor" by City and State Magazine.

Latterly, the pandemic and her husband's declining health curtailed her public appearances outside of Congress, and she faced growing misgivings about her declining cognitive abilities, raised by figures in her own party.

The youngest member of Congress, 26-year-old Maxwell Frost, described Feinstein as a "champion for Gun Violence Prevention that broke barriers at all levels of government."

"We wouldn't have had an assault weapons ban if it wasn't for Senator Feinstein and due to her tireless work, we will win it back," he posted on social media.