Republican Mitt Romney's presidential campaign says it has not been making illegal "robo-calls" to North Dakotans and says some pranksters are trying to pass themselves off as Romney supporters.

Two attorneys in Romney's campaign have sent a letter to Attorney General Wayne Stenehjem, asking him to investigate the calls. Stenehjem said Sunday his office would check out the complaint.

The letter, quoting what it said were reports from Romney supporters, linked some of the calls to a volunteer call center in Michigan that it said had done work for John McCain, the front-runner for the Republican presidential nomination.

A McCain campaign spokeswoman, Crystal Benton, said Sunday the notion that McCain's campaign was behind illegal and prank calls was "absurd."

In a letter to Stenehjem on Sunday, Trevor Potter, the McCain campaign's general counsel, said workers at the Michigan center have been making calls to North Dakotans, but only to identify potential McCain supporters and encourage them to attend Tuesday's North Dakota presidential caucus. The callers' script did not mention Romney, Potter wrote.

"I can state categorically that the McCain campaign is not doing any prerecorded or 'robocalls' into the state of North Dakota," Potter's letter says. "We have not authorized any such calls and are unaware of any being made on our behalf."

The Romney campaign's letter says the suspect robo-calls advertised themselves as being paid for by Romney's presidential campaign. North Dakota law prohibits prerecorded calls in most circumstances unless they are introduced by a live operator.

"The Romney campaign has not paid for, authorized, or made robo-calls directed at North Dakota residents in contravention of North Dakota law," the letter says.

The Romney campaign letter also says live calls have been made by people who claim to be Romney supporters but ask the people being called to attend events that do not exist. The callers "begin to laugh" when they ask the call recipient for his or her vote, the letter says.

"Clearly, our campaign is not responsible for such prank telephone calls," said the letter, which was signed by Benjamin Ginsburg, national counsel for the Romney campaign, and Kathryn Biber Chen, its general counsel.

Stenehjem said Pam Younggren, of Fargo, who is Romney's North Dakota state director, had told him recently about the calls.

"They thought maybe it was some other campaign, maybe it's some kids, maybe it's even the McCain campaign," Stenehjem said. "The whole thing, frankly, is too amateurish to believe that it comes from some candidate for president of the United States."

Stenehjem, who is a Republican, is the North Dakota state chairman for McCain's presidential campaign. Romney spokeswoman Sarah Pompei said the campaign was aware of that fact.

"We believe he'll be fair, and will look into this matter," Pompei said. "He's the attorney general of the state, and he'll perform the job that voters elected him to do."

North Dakota is one of more than 20 other states that are holding presidential caucuses on Tuesday. Stenehjem is scheduled to speak on McCain's behalf at a GOP caucus location in Williston.