The Maine Warden Service raised concerns Tuesday about a proposed expansion of background checks on private gun sales, even as national groups on both sides of the issue launched additional efforts to sway Maine voters on Question 3.

While the Warden Service did not explicitly come out against Question 3, the agency raised concerns about the ballot initiative’s enforceability and about potential impacts on sportsmen who lend guns to one another.

“The Maine Warden Service … is concerned that Question 3, if approved by the voters, will have negative impacts on some individuals who hunt and trap in Maine,” the Warden Service said in a statement. “Most importantly, this ballot question is written in such a way that it will be difficult – if not impossible – for Wardens to enforce.”

The statement came days after the National Rifle Association launched a new television ad campaign featuring Maine sheriffs urging voters to oppose Question 3. Supporters, meanwhile, were preparing to welcome former U.S Rep. Gabby Giffords – who has emerged as an influential national voice on gun control – to Portland on Wednesday to urge voters to expand background checks.

Giffords, an Arizona Democrat who was critically wounded when a gunman opened fire on a constituent event in Arizona in 2011, is scheduled to appear in Portland’s Congress Square Park from noon to 12:30 p.m.

Question 3 seeks to close a perceived loophole in Maine law that could allow individuals prohibited from possessing firearms – such as convicted felons and those with severe mental illness – to purchase guns on the private market without undergoing a background check. Supporters say the loophole feeds gun crime in Maine and other states and supports the drugs-for-guns trade.

But the ballot initiative goes further by requiring background checks prior to loaning or gifting a gun to another person – unless the recipient is a close relative.

The Warden Service’s statement echoed criticisms – and even some of the language – raised by the Sportsman’s Alliance of Maine and other opponents of Question 3 by warning that the initiative “could make criminals out of responsible firearm owners.”

“The Warden Service also has concerns about the enforcement of Question 3,” the agency said. “It would be difficult for a law enforcement officer to prove actual ownership and where the transfer occurred unless the transfer was actually observed by the officer. A routine interaction with a hunter would not generate that line of questioning unless there was reasonable suspicion that an illegal transfer had taken place.”

A representative for Mainers for Responsible Gun Ownership – the campaign behind Question 3 – said he was not surprised by the statement, but disappointed “because I believe they are misreading the initiative and its impacts.”

“Question 3 doesn’t change anything with regard to enforcement for wardens, who currently do not ask hunters if they’ve passed a background check when they encounter them in the woods,” said David Farmer, spokesman for Mainers for Responsible Gun Ownership. “It is unlikely that will change at all.”

This marks the second time in two years that a division of the Maine Department of Inland Fisheries and Wildlife has forayed into ballot question elections. In 2014, department biologists and wardens appeared in ads opposing a referendum to prohibit bear trapping as well as bear hunting using bait or hounds.

The department’s involvement in the 2014 bear referendum was criticized and resulted in an unsuccessful lawsuit against the agency. This time around, Warden Service officials said they felt an obligation to publicly state wardens’ concerns after hearing from so many sportsmen.

Colonel Joel Wilkinson, who heads the warden service, noted that the agency “handles and inspects more firearms than any other law enforcement agency in our state” and that existing laws allow them to prosecute individuals who illegally possess firearms.

“We respect this process and opinions on both sides of the issue, but feel it is imperative we inform all citizens about the potential impacts to our hunting and trapping community if Question 3 was to pass,” Wilkinson said.

Farmer, with the Question 3 campaign, pointed toward a recent study estimating that 3,000 guns a year are advertised by private sellers on just two classified listings in Maine. That study was conducted by Everytown for Gun Safety, the New York group funding much of the Question 3 campaign.

“That is a significant problem and the loophole needs to be closed,” Farmer said. “At most, what this does is it creates a minor inconvenience for people in certain circumstances. This does not turn anybody into a criminal. It simply requires a background check to sell a weapon.”

Question 3 could turn out to be one of the costliest ballot measure campaigns in Maine history. A recent Portland Press Herald poll of likely voters found that 61 percent of respondents supported Question 3.

Everytown for Gun Safety, which was created and funded by former New York City Mayor Michael Bloomberg, has already spent more than $4 million on the campaign. Opponents have spent a fraction of that sum to date, but are receiving additional help this month from the NRA.

The NRA has purchased “six figures” worth of television airtime in the Portland, Bangor and Presque Isle media markets for an ad featuring Maine sheriffs opposing Question 3, a spokesman said. In the ad, the sheriffs claim that Question 3 would send law-abiding Mainers to jail, describe the initiative as “poorly written,” and say it “is being pushed by a New York billionaire who does not care about Maine.”

Twelve of Maine’s 16 county sheriffs – all of whom are elected – signed a letter last week opposing the ballot measure. Meanwhile, the 350-member Maine Chiefs of Police Association, which includes police chiefs across the state, has endorsed Question 3 as a common-sense approach to keeping guns out of the hands of prohibited individuals. The Maine Medical Association also endorsed Question 3 this week.