The US state of Massachusetts on Tuesday elected Democrat Maura Healey as America's first openly lesbian governor, TV networks said.

Healey, 51, flipped the seat from the Republicans, comfortably defeating opponent Geoff Diehl, according to projections by NBC, Fox News and CNN.

She told cheering supporters at her election night party in Boston that she was "proud" of her historic victory.

"To every little girl and every LGBTQ person out there, you can be anything you want to be," Healey said.

LGBTQ+ rights group the Human Rights Campaign hailed her triumph.

"Massachusetts embraced a platform of equality and inclusion by electing a pro-equality champion," the organization's interim president Joni Madison, said in a statement.

Healey's win returns the state's governorship to Democrats after eight years of Republican leadership under Charlie Baker, who opted not to seek a third term.

She was on track to heavily beat Diehl, who had been endorsed by former president Donald Trump.

The result had been widely expected, with Healey -- Massachusetts' attorney general since 2014 -- comfortably ahead in the polls in the runup to the vote.

Healey will also become Massachusetts' first ever female governor. Her victory with running mate Kim Driscoll means that women will serve as both governor and lieutenant governor of a state for the first time, according to the Center for American Women and Politics at Rutgers University.

LGBTQ candidates are running in all 50 states and the capital Washington for the first time in this year's midterm election, as the community becomes an increasingly powerful voting constituency.

Democrat Tina Kotek, who is also lesbian, was bidding to match Healey's win in Oregon on Tuesday in a governor's race regarded as a toss-up.

Almost 90 percent of the LGBTQ candidates who entered this year's primary races are Democrats.