He faced harsh criticism from fellow Democrats for igniting the same-sex marriage debate in 2004 when San Francisco began granting official marriage licenses to same-sex couples.
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His words, "It's going to happen, whether you like it or not," were used to invigorate conservative voters.
Newsom is often outspoken. He describes the National Rifle Association as "morally bankrupt" and accused President Donald Trump for "operating as if he's a monarch."
Even though he was born into a prominent San Francisco family -- his father William Newsom was a California state appeals court judge with close ties to billionaire Gordon Getty -- his childhood was not easy.
Newson suffers from dyslexia, which he said made school a terrible and terrorizing experience. He opened up about the condition a few years ago on ABC7's "Beyond the Headlines," saying that he felt dumb in school.
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"I rarely read speeches and if I do they are the worst things I ever do," said Newsom, who prefers to improvise when speaking in public.
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He's had a long time to hone those skills. He was just 30 years old when he was appointed by former Mayor Willie Brown to fill a vacancy on the Board of Supervisors in 1997.
As supervisor he championed several controversial initiatives that won the backing of voters. Care Not Cash replaced cash payments to homeless in exchange for housing and social services in 2002. A year later, Proposition M banned aggressive panhandling in public areas.
Just 34 days after becoming mayor of San Francisco in 2004, Newsom directed the city clerk to issue same-sex marriage licenses. The courts debated the legality of these unions for more than a decade until the U.S. Supreme Court ruled in 2015 that they were a right guaranteed by the Constitution.
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In 2007, Newsom introduced Healthy San Francisco, which provided universal healthcare for all city residents. San Francisco was the first city in the nation to implement such a policy.
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Since he became lieutenant governor, Newsom pushed for the legalization of recreational cannabis and pushed for more strict gun control laws in the state.
He has also tussled with the White House. He recently sent a tweet to Trump, asking to debate him directly after the president criticized Newsom.
Take a look at the latest stories and videos about Gavin Newsom.