Maine and Maryland became the first states to approve same-sex marriage by popular vote, while Washington state and Colorado set up a showdown with federal authorities by legalising recreational use of marijuana.
The outcomes for those ballot measures on Tuesday were a milestone for persistent but often thwarted advocacy groups and activists who for decades have pressed the causes of gay rights and drug decriminalisation.
"Today the state of Washington looked at 70 years of marijuana prohibition and said it's time for a new approach," said Alison Holcomb, manager of the campaign that won passage of Initiative 502 in Washington state.
Colorado governor John Hickenlooper, a Democrat who opposed legalisation, was less enthused. "Federal law still says marijuana is an illegal drug, so don't break out the Cheetos or gold fish too quickly," he said.
The results in Maine and Maryland broke a 32-state streak in which gay marriage had been rebuffed by every state that voted on it. The marijuana measures in Colorado and Washington will likely pose a headache for the US Justice Department and the Drug Enforcement Administration, which consider pot an illegal drug.