Democrat Gavin Newsom has won the governor's race in California, a state that has provided some of the strongest resistance to President Donald Trump.

Newsom defeated Republican businessman John Cox in Tuesday's election to succeed outgoing Democratic Gov. Jerry Brown.

The 51-year-old was once married to former Fox News personality Kimberly Guilfoyle who is now in a relationship with Donald Trump Jr.

Guilfoyle, 49, left Fox to become vice chair of America First Policies, a pro-Trump advocacy group, and super PAC America First.

Democrat Gavin Newsom has won the governor's race in California, a state that has provided some of the strongest resistance to President Donald Trump

The 51-year-old was once married to former Fox News personality Kimberly Guilfoyle (left) who is now in a relationship with Donald Trump Jr (right)

The couple started dating following Trump Jr's split from Vanessa Trump - the mother of his five children.

Together, Trump Jr. and Guilfoyle have campaigned hard ahead of the midterm elections.

Guilfoyle, who registered as a Republican at 18, was married to Newsom when he was mayor of San Francisco. In 2004, she told Harper's Bazaar she would vote for her then husband for president.

Newsom, 51, served as lieutenant governor under Brown and has pledged to pursue universal health care and a surge in housing construction.

As San Francisco mayor, Newsom gained attention for ordering the issuance of marriage licenses to same-sex couples before it was legal.

California Democratic gubernatorial candidate Gavin Newsom hugs his wife Jennifer as he celebrates being elected governor of the state during an election night party in Los Angeles last night

He beat Trump-endorsed John Cox by 59.1 per cent to 40.9 per cent for the governorship of the nation's most populous state. He follows Democratic Gov. Jerry Brown to become the state's 40th governor.

In his victory speech he hit out at 'agents of anger determined to divide us.'

He told supporters in Los Angeles: 'It's been a tough two years, but tonight America's biggest state is making the biggest statement in America.'

'We're saying unmistakably - and in unison - that it's time to roll the credits on the politics of chaos and the politics of cruelty.

'Now is time for going far and going together. Now is the time for decency, for facts, for trust, and now is the time for truth.'

He added: 'Now is the time for leaders to lead. And to those agents of anger determined to divide us instead of unite us, it's time to pack it up and for you to pack it in.'