NEW YORK (Reuters) - Gun safety advocates are pouring tens of millions of dollars into Maine and Nevada to support ballot initiatives that would mandate background checks for gun sales in an effort to clinch state-level victories after years of failed drives in Congress. The avalanche of money spent on supporting such initiatives ahead of the Nov. 8 vote could hand gun control organizations their biggest win since they failed to secure the passage of federal legislation after the massacre of 26 children and educators at a Newtown, Connecticut, elementary school in December 2012. Voters in California and Washington state will also cast ballots on gun control initiatives, and opinion polls show the measures are likely to pass in all four states.      Everytown for Gun Safety, the gun control group founded by billionaire former New York City Mayor Michael Bloomberg, has been leading the charge, throwing its financial weight behind three of the four measures. The organization plans to spend $25 million nationwide on the issue, almost as much as the powerful National Rifle Association has spent on television advertising for Republican presidential candidate Donald Trump. The right to own firearms is protected by the Second Amendment of the U.S. Constitution, and any efforts to restrict that guarantee are fought vehemently by gun rights advocates, who are highly influential within the Republican Party. Victories by gun control groups in Maine and Nevada would mean that half of the U.S. population would live in states with expanded background checks, including private sales not involving a licensed dealer. Gun control supporters say that would be a symbolic and strategically important threshold. Opponents of the measures say the laws are poorly written, would do little to combat crime and would punish law-abiding gun owners. A University of New Hampshire poll in late October found 52 percent of Maine residents support the measure and 43 percent oppose it. In Nevada, two polls in late October found a 16-percentage-point margin and a 25-point margin in favor of the initiative, respectively. The measures come in a year that voters around the United States will weigh in on 71 citizen-submitted ballot initiatives on topics ranging from marijuana legalization to raising the minimum wage, the largest number in a decade, according to Ballotpedia.org, a website that tracks voting data. Everytown described its strategy as part of an effort to copy the state-by-state tactic that helped legalize gay marriage across the United States, with Congress unwilling to pass universal background check legislation. “One of the great advantages of going directly to the electorate is that you can go around the legislative bodies and make law,” said Zach Silk, a consultant who has worked with Everytown in Nevada and previously helped oversee Washington state’s 2014 successful background-check ballot initiative. Nevada in particular has become an expensive battleground. Bloomberg has personally donated nearly $10 million to the effort there, and Nevadans for Background Checks had collected $14.3 million as of Oct. 18. That is nearly triple the $4.8 million that the leading opposition group, NRA Nevadans for Freedom, had received, all from the NRA. In Maine, supporters of background checks have outspent opponents by more than 5-to-1, with the Everytown-backed Mainers for Responsible Gun Ownership Fund reporting $5.3 million in contributions as of last week. The NRA did not respond to a request for comment on its campaign spending decisions. Critics have accused Bloomberg of trying to buy victory.  “This law is convoluted and way overregulatory,” said David Trahan, the executive director of the Sportsman’s Alliance of Maine and a former state legislator. “It reflects the person funding this initiative, Michael Bloomberg. His solutions are overkill, and they border on social engineering.” But Everytown’s executive director, John Feinblatt, said it was the gun lobby that has tried to take power away from the citizenry. “While the NRA can control legislators and the government, they can’t control the people,” he said. 