Democrats have won key elections in New York City and the state of Virginia, in the first major round of polls since United States President Barack Obama's re-election in 2012.
Tuesday's races are seen as an early test of both parties' strengths ahead of congressional mid-term elections next year.
Bill de Blasio has been voted mayor of New York to succeed Michael Bloomberg - the first time that a Democrat has captured City Hall in two decades.
With 14 precincts reporting in New York, Mr de Blasio secured 73% of the vote, convincingly defeating his Republican rival Joe Lhota, who polled 24%, the BBC reports.
In his victory speech, Mr de Blasio said the poll showed that America's largest city had chosen "a progressive path", and he promised to make fighting income inequality his top priority.
"I will never stop fighting for the city I love, the city we all love so much. And I will never forget that as mayor, I work for you," he told supporters.
Mr de Blasio is the city's first Democratic mayor since 1993. He ran Hillary Clinton's 2000 Senate campaign, and is seen as one of the most liberal politicians who has run for mayor in decades.
In another closely fought battle, Democrat Terry McAuliffe defeated his opponent Republican Ken Cuccinelli to become Virginia's next governor.
With results from 97% of precincts so far counted, Mr McAuliffe has polled 47% to his opponent's 46%.
Mr McAuliffe is a businessman and veteran Democratic party fundraiser. He has close ties to former President Bill Clinton and former Secretary of State Hillary Clinton, serving as chairman of her 2008 presidential campaign.
Mr Cuccinelli, the Virginia attorney general, has angled for the support of the hardcore conservative Tea Party movement of Republicans.
In New Jersey, Republican Chris Christie was easily re-elected as governor against Barbara Buono.
Colorado votes on splitting state
Voters in Colorado are casting their ballot over whether the state should be split into two - creating a 51st American state.
Several US states have ballot measures in front of voters on Tuesday, Radio New Zealand's correspondent reports from Denver.
In heavily conservative parts of northern Colorado, voters are being asked if their part of the state should break away from Colorado. The move began as a protest against tough new gun control measures.
Another issue facing Colorado voters is how to tax marijuana. The ballot asks whether the state should adopt a 15% excise tax on pot and an extra 10% on sales tax is also being proposed to fund drug enforcement.