Voters in New York picked Bill de Blasio over Michael Bloomberg to be the city's first Democratic mayor in a generation, in one of a handful of elections seen as having considerable national importance.
New Jersey's Republican Governor Chris Christie handily defeated his Democrat challenger, earning a second four-year term in a race some pundits said would serve as a platform for a future presidential bid.
Another key race, for the governorship of the southern state of Virginia, turned out to be be a nail-biter, with media reports declaring businessman Terry McAuliffe victorious by the narrowest of margins.
President Barack Obama warned against making early election calls.
"Never predict elections," he said. "That's a losing proposition."
That word of caution proved wise in Virginia, where McAuliffe, a Democrat, had been predicted to make short work of his Republican opponent, State Attorney General Ken Cuccinelli, in a race that turned out to be far tighter than originally predicted.
Early election results had shown McAuliffe and Cuccinelli tied at 47 per cent each shortly after polls closed.
Yesterday's vote is the first major round of balloting in the United States since Obama won a second term in the White House last year.
Christie cruised to a landslide victory on a wave of popularity, but at odds with his party at the national level.
He is increasingly seen as a contender to win the Republican nomination for the White House in 2016 given his pragmatism, charisma and ability to command cross-party support.
Meanwhile, the race in the Big Apple -- in which de Blasio long had been tipped as the heavy favorite to replace billionaire Michael Bloomberg -- was one of several seen as a barometer of public opinion ahead of congressional elections in 2014.
De Blasio, 52, promises a new style in a city transformed by 12 years of tough love under Bloomberg, who is stepping down after a record three terms.
He left Republican rival Joe Lhota trailing in the dust in the biggest city in the United States by tapping into the worries of the economically vulnerable middle class.
The city of 8.3 million has six times as many Democratic voters as Republicans, yet a win for de Blasio would make him the first Democrat elected mayor since 1989.
"I think the people of this city know that so many New Yorkers are struggling just to make ends meet," he said after voting with wife Chirlane in his Brooklyn neighborhood, accompanied by their