BOZEMAN, Mont. -

A new ad attacks Republican Senate candidate Denny Rehberg on his support for the National Security and Federal Lands Protection Act. Rehberg is a co-sponsor of the bill.

"Congressman Dennis Rehberg would let the Department of Homeland Security monitor and control access on public lands -- millions of acres," the ad says.

It's key claim -- that the legislation would allow the government to conduct surveillance on federal land -- is true.

I examined the language in the bill and found it would give the Border Patrol the right to ignore dozens of federal laws to "achieve operational control" within a hundred miles of the Mexican and Canadian borders -- adding up to more than 477 million acres.

The ad supports Libertarian candidate Dan Cox. But the Rehberg campaign claims it's secretly funded by supporters of Democrat Jon Tester,and that it's trying to split the conservative vote.

The $500,000 spot was paid for the Montana Hunters and Anglers Leadership Fund, which shares a name with the Montana Hunters and Anglers Action Group. We couldn't confirm that the two groups are related, but the action group's president and treasurer did donate money to the leadership fund. Repeated attempts to contact them were unsuccessful, so I decided to dig deeper.

According to Federal Election Commission data, Montana Hunters and Anglers President Land Tawney and Treasurer Barrett Kaiser gave $2,850 to Tester between 2011 and 2012.

Kaiser also worked on Tester's successful Senate campaign in 2006.