BANGOR, Maine — In a major upset for Maine’s gun rights grass-roots and over a campaign apparatus that steeply outraised them, voters rejected an expansion of background checks on private gun sales and transfers on Tuesday.

The win is a blow to Everytown for Gun Safety, a national group founded by Michael Bloomberg, the billionaire ex-mayor of New York City, which funded virtually all of the nearly $6 million campaign in Maine. A similar race in Nevada funded by the group hung in the balance early Wednesday morning.

By any measure, it’s a stunning victory for what David Trahan, the executive director of the Sportsman’s Alliance of Maine, called a group of “ragtag volunteers” that shows “you can win on an issue without money” — though they had $1 million with some support from the National Rifle Association — and “you can do things at the grass-roots level.”

“I’ve been involved in a lot of campaigns over the years, but this one I’ll never forget because it was truly a grass-roots campaign,” he said.

David Farmer, a spokesman for Mainers for Responsible Gun Ownership, said the campaign wasn’t conceding with 75 percent of precincts reporting in Maine.

“We’re going to wait to see all the votes counted,” he said.

The law would have required background checks before private gun sales or transfers, regulating the types of sales common in the classified ad catalog Uncle Henry’s or online venues such as Armslist and making a first conviction a misdemeanor and another a felony.

Nationally, the expansion of background checks polls well, often above 80 percent. But a Yale University study earlier this year attributed some of that to misinformation, saying that more than two-fifths of Americans think universal background checks are already the law.

Opponents reinforced that perception: In a TV ad, Franklin County Sheriff Scott Nichols said background checks are “already” law and that the law wouldn’t stop criminals, but it would rather create them out of otherwise law-abiding Mainers.

He and other opponents focused on the law’s limited exceptions, which allow for transfers without background checks between family members, while people are hunting or sport shooting together and other circumstances.

But law enforcement has long flagged gun trafficking as an issue here. A gun bought without a background check killed Portland resident Darien Richardson in 2012 and was used later to kill Serge Mulongo, another Portlander. Richardson’s parents were among the most visible backers of Question 3.

But the campaign was always up against elements of Maine’s culture. In 2011, it had the fifth lowest rate of firearm use in crimes in the U.S. at 14.2 percent, less than half the national average, according to the University of Southern Maine.

Rural Maine led the way in opposing the question, mirroring the 2004 and 2014 votes against banning the three main methods of bear hunting in Maine, with similar constituencies coming down of both sides of this question.

In Unity on Tuesday, Bill Allen, a retired Maine Warden Service lieutenant who voted for Democrat Hillary Clinton, said Question 3 wasn’t necessary and wouldn’t prevent criminals from getting guns.

“I don’t think it’s going to accomplish what they want it to accomplish,” he said.