By Joseph Ax

NEW YORK (Reuters) - Gun control-related ballot measures in four states are expected to pass on Tuesday, opinion polls showed, after gun safety advocates poured a massive amount of money into backing the initiatives.

In Maine and Nevada, residents are voting 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 will consider allowing judges to bar dangerous people, such as accused domestic abusers, from possessing guns. 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 it.

The votes in 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 have said the laws are not clear and would do 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."

Proponents have said background checks would save lives.

Jennifer Crowe, a spokeswoman for the pro-initiative campaign in Nevada, said research had shown that nearly one in 11 people in that state who purchased guns online would have failed 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.

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Shares of gun makers rose on Tuesday as investors wagered that Democrat Hillary Clinton would win the presidency. Smith & Wesson Holding Corp closed up 2.15 percent at $28.45 and Sturm Ruger & Company Inc finished up 0.86 percent at $64.40.

A Clinton presidency would likely lead to a surge in gun purchases out of fear she might try to enact stricter gun policies, said Chris Krueger, senior research analyst at Lake Street Capital Markets.

(Reporting by Joseph Ax; Editing by Jonathan Oatis, Toni Reinhold)