Groups on both sides of a ballot initiative to expand background checks on gun sales are buying television airtime and mobilizing their bases ahead of what could be one of the costliest referendum campaigns in Maine history.

The organization Mainers for Responsible Gun Ownership has reserved roughly $2 million in commercial slots on television stations statewide between Labor Day and Election Day. On Wednesday, the group launched its second television ad – featuring former U.S. Attorney Paula Silsby – in support of the initiative to require criminal history background checks for most private gun sales and gun transfers in Maine.

The campaign has received the vast majority of its funding so far from Everytown for Gun Safety, the gun control group created by former New York City Mayor Michael Bloomberg. Everytown is spending millions of dollars on high-profile ballot campaigns in Maine and Nevada this fall. And in another sign of the national focus on the Maine race, the daughter of a Connecticut elementary school principal shot to death while confronting the gunman who ultimately killed 20 children is spending several days in Maine this week talking to campaign volunteers.

“The opposition is going to get ugly and going to get mean,” Erica Smegielski, daughter of Sandy Hook Elementary School Principal Dawn Hochsprung, told about a dozen volunteers in Portland on Wednesday evening. “But what you have on your side is the simple fact that you are right.”

Opponents of Question 3 on the November ballot also are getting assistance from an out-of-state heavy hitter – the powerful National Rifle Association. The Virginia-based NRA hit the television airwaves this week with an ad accusing “New York billionaire Michael Bloomberg of spending $3 million to try and boss Mainers around.”

An NRA spokesman did not have a dollar figure for the ad buy on Wednesday, but described it as an “initial” purchase and said the commercials are running on cable stations statewide.

David Trahan, executive director of the Sportsman’s Alliance of Maine, said he is working hard to raise money but acknowledged that “we don’t have a choice” but to rely on the NRA for financial support. That’s because Maine’s sporting community is “tapped out” after successfully fighting a 2014 referendum to ban bear hunting with dogs and bait, he said.

Opponents also claim Maine, with its rich hunting tradition, has a long history of private gun sales and one of the lowest crime rates in the country. Trahan and others also say Question 3 will only hurt law-abiding Maine gun owners, forcing them to get background checks just to loan a gun to a friend and making them vulnerable to prosecution.

“The law is so narrow it sets up booby traps for law-abiding citizens who have no intention of committing a crime,” Trahan said.

Question 3 would expand the scope of Maine’s background check requirements for gun sales and transfers, seeking to accomplish at the state level what gun control advocates have failed to move through Congress.

Under current state law, only federally licensed firearms dealers such as gun store owners are required to run the names of would-be buyers through the FBI’s National Instant Criminal Background Check System. Anyone purchasing a gun from a private seller is not required to undergo a background check.

Critics contend that loophole allows people prohibited from owning firearms to easily obtain them without detection. They also say Maine’s looser gun laws make the state a source of black market guns that fuel violence in states such as Massachusetts, New York and Connecticut that have tighter gun laws.

The ballot initiative would mandate background checks for all private sales or transfers, with exceptions for transactions between family members, to law enforcement or for some “temporary transfers” for hunting or personal defense.

Maine has a robust, unregulated private marketplace for guns, often carried out online or through the popular weekly classified shopper Uncle Henry’s. In an analysis released last week, Everytown said that unlicensed gun sellers placed more than 8,000 gun ads in Uncle Henry’s over a 44-month period, while another popular site carried more than 500 items posted by unlicensed Maine sellers.

On Thursday, the Maine Chiefs of Police Association is expected to endorse the campaign to expand background checks. And campaign organizers point to polls showing that nearly 7 in 10 respondents in Maine said they would support requiring background checks on most firearms transactions.

The background checks expansion is one of five ballot initiatives plus a bond measure that will appear on a Nov. 8 ballot already crammed with elections for the Legislature, Congress and the presidency. Several of the other referendum questions – such as the proposals to legalize marijuana for recreational use and to raise Maine’s minimum wage – are expected to draw significant campaign spending, although the gun issue appears likely to be the costliest.

Everytown for Gun Safety had funneled roughly $3 million into the campaign as of mid-July, a figure that includes collecting signatures needed to qualify for the ballot. That is already more than the $2.5 million spent by supporters of the failed 2014 bear hunting referendum and is likely to compete with the $4.5 million spent on the unsuccessful 2009 effort to legalize same-sex marriage.

Tears flow as Sarah Rawlings of the Yes on 3 campaign listens to Erica Smegielski, the daughter of the Sandy Hook school principal killed by a gunman in 2012 along with 20 students. Smegielski’s visit is one sign of the national focus on Maine’s gun sales vote.

Smegielski, who works with campaigns around the nation as a full-time staffer for Everytown for Gun Safety, recalled Wednesday how she learned about the shooting at her mother’s elementary school on the news and waited hours for confirmation of what she and other family members suspected. After weeks of debilitating sorrow, Smegielski said she was galvanized into action after hearing U.S. senators say they didn’t believe a Sandy Hook-inspired gun control bill even deserved a vote.

Gun control advocates failed to win the 60 votes needed to move the measure – known as the Manchin-Toomey bill – forward in the Senate in April 2013. But Everytown has since shifted its focus onto the states, starting with Connecticut and Washington.

Erica Smegielski, the daughter of a school principal killed by an invading gunman, speaks Wednesday to Yes on 3 staffers.

“I’m still riding the wave of 2014 when Washington state passed background checks because the people wanted it,” Smegielski told volunteers. “And now you guys are doing that.”

Mainers for Responsible Gun Ownership already is organizing phone banks to call voters and sending volunteers out to farmers markets, fairs and other events. Opponents, meanwhile, are trying to raise money and organize their campaign to fight against a ballot initiative they see as misguided and promoted by out-of-state interests.

“At the grassroots level, we are doing great,” Trahan said. “We had 5,000 lawn signs and, when we let people know we had them, they were gone within the week.”