(CNN) - A Republican Senate candidate in Montana is facing questions over comments he made suggesting he knew of the National Rifle Association's plans to be involved in his race -- a move that experts say could violate campaign finance laws meant to prohibit campaigns from coordinating with outside groups.
The comments, recorded on audio first published by the Daily Beast and subsequently obtained by CNN, features the candidate, Matt Rosendale, talking about Chris Cox, a chief strategist with the NRA's Institute for Legislative Action, and the group's plans to support his challenge against Democratic Sen. Jon Tester, a red-state incumbent up for re-election in the fall.
The Tester-Rosendale race is one of the most closely watched Senate contests in the country, with Rosendale looking to capitalize on Donald Trump's popularity in a state the President won by 20 percentage points in 2016.
"Outside groups are spending on your behalf?" an unidentified questioner asks Rosendale in the audio.
"Yes," Rosendale answers. "So the Club for Growth has already started. There's another group that has already started. I can't even remember the name of it now. They just started recently. So outside groups already started to come in. And I fully expect that the US Chamber's going to come in, and I fully expect the NRA is going to come in. I think both of them will be coming in, probably, right here in August sometime."
"This is a big race for the NRA," the questioner adds.
"Big. The Supreme Court confirmations are big -- that's what sent the NRA over the line, because in '12, with Denny (Rehberg), they stayed out. They stayed out of Montana. But Chris Cox told me, he was like, 'We're going to be in this race.' "
According to Federal Election Commission filings, the NRA-ILA spent over $400,000 on TV ads opposing Tester in early September.
At issue: The NRA-ILA, which is a 501(c)(4) nonprofit, is prohibited from coordinating independent expenditures -- like the ads against Tester -- with a federal campaign such as Rosendale's.
Tester's campaign seized on the news to cast Rosendale as an outsider. Chris Meagher, the communications director for Tester's campaign, said, "This audio raises serious concerns about potential illegal coordination between Matt Rosendale and an outside, dark money group coming into Montana to support him."
Both Rosendale's campaign and the NRA-ILA strenuously denied violating campaign finance rules, and said they had only discussed an endorsement.
The campaign acknowledged that it's Rosendale who's speaking on the tape.
The campaign's statement said that "this is amusing desperation on Jon Tester's part and it's completely baseless. The only thing this audio proves is that Matt sought the endorsement of the NRA -- and we're proud to have it. Matt and the NRA have never discussed anything beyond the organization's membership and endorsement process."
Jennifer Baker, director of NRA-ILA public affairs, said that "at no time did NRA-ILA discuss any communications or activities beyond our membership with Matt Rosendale or his campaign. Any assertion otherwise is completely false."
According to FEC regulations, there is a "three-prong test" to determine illegal coordination: payment, content and conduct. All three prongs must be met for coordination to be established.
Christian Hilland, a press aide with the FEC, noted the commission could not comment on any potential enforcement matters, but explained that, broadly, the coordination rule "comes into play when an individual or political committee pays for a communication that is coordinated with a candidate or party committee. If coordination of the communication occurs, the result is an in-kind contribution to the federal candidate."
Larry Noble, general counsel of the FEC for 13 years, told CNN that Rosendale's comments "clearly raise questions about whether the NRA illegally coordinated their ad campaign with Rosendale."
"In his comments, Rosendale references a conversation he had with Chris Cox in which he was told that the NRA would be involved in his race," Noble said. "If the NRA and Rosendale's campaign discussed the content, issues or strategy involving the ads, or if Cox approved of the NRA's ad campaign, the money spent by the NRA-ILA in coordination with Rosendale could constitute an illegal contribution to his campaign."
"Not only does this raise questions under the federal campaign finance laws, but it could also raise questions under the Internal Revenue Code. Rosendale's comments are clearly sufficient to warrant the FEC investigating the matter," Noble added.
Brendan Fischer, the federal reform program director at the Campaign Legal Center, said his group planned to file a complaint with the FEC about the Rosendale comments.
"If the outside group goes to the candidate and says, 'We're going to run ads during this period, with this message,' and the candidate assents to that, that too is considered coordinated," Fischer said.
Anne Feldman, a spokeswoman for End Citizens United -- a political action committee focused on campaign finance restructuring and overturning Citizens United, the 2010 Supreme Court decision -- said the audio "definitely raises very real concerns about illegal coordination between Rosendale and the NRA-ILA."
"The NRA is spending this money because they know they can influence Rosendale to vote their way," Feldman said. "They are telling him they will spend, that's just illegal coordination."
The Rosendale campaign referred CNN to Brad Smith -- a Republican former FEC commissioner nominated by President Bill Clinton -- who said he had listened to the audio and had heard "absolutely nothing wrong."
"That is not coordination in any way," he said. "Just telling people we expect to be involved in your way is not coordination in any way."
He added: "I just didn't hear anything that even remotely would have made me concerned as an attorney."