Audio recordings of Sheehy's racial comments were obtained and published by Char-Koosta News, the official publication of the Flathead Indian Reservation.

A Sheehy campaign spokesperson did not dispute the authenticity of the recordings, which the tribal newspaper said came from fundraising events held in Montana last November.

Sheehy is heard commenting in one of the recordings that his ranching partner is a member of the Crow Tribe with whom Sheehy ropes and brands cattle on the tribe's southeastern Montana reservation.

"Great way to bond with all the Indians, to be out there while they're drunk at 8 a.m.," Sheehy says.

In another recording, he describes riding a horse in the parade at Crow Fair, an annual gathering on the reservation that includes powwows, a rodeo and other events.

"If you know a tough crowd, you want to go to the Crow res," Sheehy says. "They let you know whether they like you or not -- there's Coors Light cans flying by your head riding by."

Sheehy is challenging three-term incumbent Democrat Sen. Jon Tester in one of the most closely-watched congressional races in the nation. A Republican victory could help decide control of the closely divided Senate.

Montana has seven Indian reservations and almost 70,000 Native Americans, representing about 7% of its total population. It's a voting block that's long been considered Democratic-leaning, but Montana Republicans in recent years have courted tribal leaders hoping to gain their support in elections.

The Rocky Mountain Tribal Leaders Council, which represents 11 tribes and First Nations in the western U.S. and Canada, said Sheehy's comments perpetuated stereotypes about Native Americans.

Council Chairman Bryce Kirk asked Sheehy to formally apologize in a Tuesday letter to the campaign obtained by The Associated Press.

"You ask for our votes and then you go to your fundraiser, ironically with alcohol flowing and laughter at our expense behind closed doors, and you insult us with a stereotype that only seeks to severely diminish and dishonor our people," Kirk wrote. "The Crow people are not your punchline. Native Americans are not your punchline."

Sheehy spokesman Jack O'Brien said Wednesday that the Republican knows members of the Crow Tribe and visits the reservation to work cattle with them.

"He works with them, he brands with them," O'Brien said.

O'Brien did not say if Sheehy would apologize or otherwise respond to the tribal leaders' letter.

"What folks are insinuating about him, that's just not who he is," he said.

Crow tribal Chairman Frank White Clay did not immediately respond to a message left with his office seeking comment.

Char-Koosta News editor Sam Sandoval said Sheehy's campaign had not responded to his outlet's queries about the recordings, which he said came from a credible source who wanted the comments publicized in a tribal newspaper.

"For a lot of tribal people, having that statement out there, saying they're drunk at 8 o'clock in the morning, it really hits a sore spot that Natives have been working to change for years," Sandoval said.