HELENA - Dennis McDonald, a Democratic rival of U.S. Rep. Denny Rehberg, on Monday accused the congressman of being drunk when he rode as a passenger in a powerboat that later crashed into the rocky shore of Flathead Lake, seriously injuring all five aboard.

However, toxicology and legal experts cautioned that the method McDonald used to calculate Rehberg's alleged intoxication produces, at best, an estimate, and is not used in Montana courts as proof that someone was drunk.

Rehberg and two staff members were passengers in a boat piloted by state Sen. Greg Barkus, R-Kalispell, that crashed into the cliffs at Wayfarers State Park near Bigfork on Aug. 27.

Barkus' wife Kathy was also in the boat.

Rehberg was not driving the boat, but released the results of his blood-alcohol content tests, taken in the emergency room at the Kalispell hospital three hours after the crash. His blood-alcohol content was 0.05 percent, below Montana's legal limit of 0.08 percent.

McDonald, a Melville rancher, former Montana Democratic Party chairman and one of two Democrats challenging Rehberg in next year's congressional race, on Monday accused Rehberg of being drunk that night.

He said Rehberg would have been over the legal threshold of intoxication when he decided to get in Barkus' boat.

"Despite his best efforts to deceive the public on how much he had drunk that evening, Mr. Rehberg was legally intoxicated when he made the decision to get on the boat and led his staffers with him," McDonald wrote.

A spokesman for Rehberg dismissed McDonald's comments as a "classless" attack designed to revitalize a faltering campaign.

McDonald said the concentration of alcohol in the blood decreases at a rate of 0.015 percent per hour. Rehberg's level was measured three hours after the accident, so he would have had a blood-alcohol level exceeding 0.10 percent, or greater than the legal limit, McDonald said.

McDonald said Rehberg would have been too intoxicated to judge whether Barkus, who has not released his blood-alcohol content, was likewise drunk.

"These two drunks made a series of bad decisions and five people were seriously hurt, including themselves," McDonald wrote.

David Andrenyak, a research toxicologist at the Center for Human Toxicology at the University of Utah's College of Pharmacy in Salt Lake City, said an average person will metabolize alcohol in the blood at a rate of about 0.015 percent an hour. However, he said, there are many factors in deciding if someone is "drunk."

"People will metabolize alcohol at different rates," he said. Using the formula to "prove" drunkenness is "something that is looked at very cautiously and very carefully."

Missoula County Attorney Fred Van Valkenburg, a former Democratic state lawmaker, said he doesn't believe the 0.015 percent multiplier produces "proof" that someone was intoxicated.

"I think you're stretching it when you start trying cases in that fashion," he said.

Rehberg has said he had less than two pints of beer at a Lakeside restaurant during dinner, before getting in the boat. He said he wasn't paying attention to how much Barkus had to drink, but said the state lawmaker did not appear intoxicated.

Barkus has not been charged with any crime, although local authorities have said Barkus was drinking that night and they are investigating the incident.

Rehberg campaign spokesman Tyler Matthews attacked McDonald for using the crash to raise money.

"Mr. McDonald's continued use of this tragic accident to raise money for his faltering political campaign is classless, but probably not all that unexpected," he said. "After all, he was a longtime friend and employee of confessed murderer and Mafia boss Jimmy 'The Weasel' Fratianno."

McDonald, formerly a lawyer in California, once represented Fratianno in a years-long case in which Fratianno served as a state's witness against other Mafia members.

Tyler Gernant, a Missoula lawyer and the other Democrat who has announced for Rehberg's seat, criticized both men on Monday.

"One Dennis is demanding an apology from the other Dennis for having a couple of drinks," Gernant wrote in a written statement. "The other Dennis turns around and releases his attack dogs on Dennis for being a mob lawyer from California. This kind of petty political posturing does nothing to help the 158,000 Montanans currently without health insurance."