Gun control-related ballot measures in four states are expected to pass on Tuesday, opinion polls show, after gun safety advocates poured a massive amount of money into backing the initiatives.
In Maine and Nevada, residents will vote on whether to mandate universal background checks for firearm sales, including private handgun transactions.
If those two measures pass, half of all Americans would live in states that have such expanded checks. Eighteen states and Washington, D.C., have already approved similar laws.
Voters in Washington state, meanwhile, will consider allowing judges to bar people from possessing guns if they pose a danger to themselves or to others, such as accused domestic abusers. In California, a referendum would ban large-capacity ammunition magazines and require certain people to pass a background check to buy ammunition.
The U.S. Constitution's Second Amendment protects the right to bear arms, and gun rights advocates fiercely contest any attempt to restrict that freedom.
The votes in Republican-leaning Maine and Nevada represent a key test of the gun control movement's decision to turn to a state-by-state strategy after efforts to pass nationwide legislation failed in Congress.
Opponents in Maine and Nevada say the laws are confusingly written and would burden legal gun owners while doing nothing to stop criminals.
"We know today that the place where criminals are getting guns, the black market, they aren't subjecting themselves to background checks," said Ryan Hamilton, a spokesman for the National Rifle Association-backed opposition in Nevada. "It doesn't target criminal behavior, it targets law-abiding behavior."
But proponents say background checks are widely backed by the public and would save lives.
Jennifer Crowe, a spokeswoman for the pro-initiative campaign in Nevada, said research had shown nearly one in 11 people who purchased guns online would have been barred from doing so by a background check.
"We have this huge online marketplace that we know criminals are using to get guns," she said.
Everytown for Gun Safety, the gun control group founded by billionaire former New York City Mayor Michael Bloomberg, has spent tens of millions of dollars in Washington state, Nevada and Maine, while the National Rifle Association has focused much of its spending on supporting Republican presidential nominee Donald Trump.
In Nevada, the most expensive contest, the background check campaign collected more than $14 million, much of it from Bloomberg. The NRA devoted $4.8 million to fighting the measure.