HELENA — A national political group led by top GOP operatives unleashed another TV ad Thursday criticizing U.S. Sen. Jon Tester, alleging that the Montana Democrat sided with Obama administration efforts to regulate farm dust.

Tester's campaign quickly denounced the segment from Crossroads Grassroots Policy Strategies as "more lies" from his GOP opponents.

"Jon never voted to regulate farm dust," said his campaign manager, Preston Elliott. "As the Senate's only farmer, he already got the Environmental Protection Agency to back off this ridiculous proposal."

The 30-second spot, which began airing Thursday in Billings, Missoula, Butte, Helena and Great Falls TV markets, says Tester "voted against preventing Obama's EPA from being able to regulate Montana farmers' dust."

"That's not the Montana way," the narrator on the ad says. "Tell Tester, on jobs, he needs to work for Montana, not Obama."

The ad then flashes a toll-free number for Tester's Washington, D.C., office and shows a picture of Tester standing with President Barack Obama.

Crossroads GPS is a sister organization of American Crossroads, a political action group founded by Karl Rove, the former White House deputy chief of staff under President George W. Bush. Crossroads GPS officers include former staffers for the Republican National Committee and the Bush administration.

The ad on "farm dust" refers to a procedural vote last month in the U.S. Senate on a bill to help the Obama administration fight Chinese manipulation of its currency.

The Senate had voted to end debate on the bill and minority Republicans wanted to offer a number of unrelated amendments. Senate Majority Leader Harry Reid, D-Nev., asked the Senate to block any amendments, which he considered another delaying attempt.

The Senate voted 51-48 to uphold Reid's request, with Tester in favor.

One of the amendments that Republicans wanted to offer would have blocked EPA regulations on farm dust.

Tester's office noted Thursday that he signed a letter to the EPA in February opposing the regulation of farm dust and that the EPA decided in October not to increase regulation of particulate matter, including farm dust.

"Only Washington insiders like Dennis Rehberg and Karl Rove would have you believe that a procedural vote on Chinese currency somehow relates to farm dust," Elliott said Thursday. "Their dots will never connect to the truth, so instead they're serving us more lies."

Rehberg, Montana's U.S. House representative and a Republican, is challenging Tester in the 2012 election.

Crossroads GPS produces what are called "issue ads," that usually attack a political figure and urge viewers to "call" the target of the ad and express their dissatisfaction. Because the ads don't specifically tell someone to vote for or against a candidate, their sponsor doesn't have to reveal its financial donors.

The group said it bought $157,000 in TV time in Montana for the Tester ad, as part of a nationwide buy for similar ads. The other spots are running in Missouri, Nebraska, Virginia and Massachusetts, attacking Democratic U.S. Senate candidates in those states.

The Montana ad is the third spot that Crossroads GPS has run this year going after Tester.