Democrats take New York and Virginia as Republicans win New Jersey
Moderate Republican New Jersey Governor Chris Christie has won a landslide re-election, a possible launch pad for a 2016 White House run.
In Virginia, Terry McAuliffe - a key Hillary Clinton ally - narrowly beat a Republican to become governor of the pivotal presidential swing state.
Bill de Blasio's thumping victory in New York makes him the city's first Democratic mayor-elect in two decades.
Tuesday's results point the way to next year's US midterm elections.
New Jersey's straight-talking Governor Christie brushed aside his Democratic challenger, state senator Barbara Buono, by 60.5% to 38%, retaining a handy platform for a potential presidential bid.
Christie's bipartisan appeal
His ability to attract the support of Democrats, independents and minorities makes him a tantalising prospect for the ideologically split Republican party ahead of 2016.
"We stand here tonight showing that it is possible to put doing your job first," said Mr Christie in his victory speech, "to put working together first, to fight for what you believe in yet still stand by your principles and get something done for the people who elected you."
The governor, known for his readiness to work with Democrats in a left-leaning state, won much praise for his handling of the carnage wrought on the region by super-storm Sandy one year ago.
However, fellow Republicans at a national level lambasted him for his embrace of Democratic President Barack Obama as he visited Jersey Shore to see the devastation.
In the Virginia governor's race, Mr McAuliffe eked out a smaller-than-expected, 48% to 45.5% win against socially conservative Republican Ken Cuccinelli.
"Over the next four years most Democrats and Republicans want to make Virginia a model of pragmatic leadership," said Mr McAuliffe, a Democrat taking charge in a state with a Republican-led legislature. "This is only possible if Virginia is the model for bipartisan co-operation."
During the campaign, Mr McAuliffe had emphasised his opponent's tea party support. The hardline wing of the Republican party was widely blamed for last month's government shutdown, which temporarily laid off many of the federal workers who populate Virginia's suburbs.
Mr Cuccinelli, the state's outgoing attorney general, had stressed his Democratic rival's championing of the Obama healthcare law, whose troubled rollout has been a White House public relations disaster.
A Libertarian candidate who won nearly 7% of Virginia's vote may have cost the Republican the race, say analysts. Mr Cuccinelli was also vastly outspent by his Democratic opponent.
Marijuana and secession
Mr McAuliffe is a veteran Democratic party fundraiser who was chairman of former Secretary of State Hillary Clinton's 2008 presidential campaign.
His capture of the governor's mansion in a crucial presidential swing-voting state could help plot a road map to victory for her should she run again for the White House, say analysts.
Meanwhile, Mr de Blasio will succeed New York Mayor Michael Bloomberg to become the first Democratic mayor of America's largest city since 1993.
He secured a whopping 73% of the vote, compared with 24% for his Republican rival, Joe Lhota.
In his victory speech, Mr de Blasio said the result showed that America's largest city had chosen "a progressive path", and he promised to make fighting income inequality his top priority.
In other elections across the US on Tuesday:
- Colorado voted to tax marijuana sales; Portland, Maine, became the first US East Coast city to legalise use of the drug
- Six of 11 counties in Colorado passed symbolic votes to secede from the state
- Republican infighting played out in Alabama where a candidate backed by business interests beat a tea party rival in a primary vote
- Mike Duggan became the first white mayor-elect of Detroit since the 1970s
- A Seattle suburb raised its minimum wage to $15 an hour; New Jersey increased it to $8.25 an hour
- Washington state voters rejected a measure to label all genetically-modified food
Tuesday's off-year polls could be seen as a litmus test of the parties' support ahead of the 2014 midterm elections.
Next year's votes will decide the make-up of the House of Representatives, one-third of the Senate, and governorships in more than half the states.
Mr Obama's Democratic party controls the US Senate, while Republicans hold sway in the House of Representatives.
Now in his second term, Mr Obama will vacate the presidency in 2017.
- New York City Mayor
Joseph Lhota (Republican): 24% - Bill de Blasio (Democrat): 73%
- New Jersey Governor
Chris Christie (Republican): 60.5% - Barbara Buono (Democrat): 38%
- Virginia Governor
Ken Cuccinelli (Republican): 45.5% - Terry McAuliffe (Democrat): 48%