The campaign for Colorado's open Senate seat has quickly turned ugly since last month's primary, with TV attack ads focusing on an unusual issue for both Republican Peter Coors (search) and Democrat Ken Salazar (search): the environment.

A Virginia pro-business group fired one of the first shots in the tight race that will help decide control of the Senate. Its ad said Salazar, the state's former natural resources chief and current attorney general, settled for less than $30 million from the owner of a mine that unleashed one of Colorado's worst environmental disasters. The spot said the disaster will cost an estimated $240 million to clean up.

Salazar denounced the claims as "straight-out lies," Colorado newspapers editorialized against it and the League of Conservation Voters (search) aired its own spot defending him.

Americans for Job Security President Mike Dubke said his group's ad was meant to show that Salazar "fleeced the taxpayers."

"A lot of folks thought it was an environmental ad. From my point of view, it's all about taxes," Dubke said of the ad that cost about $900,000 to run statewide. "It's a litmus test for what people project onto it."

Coors, on leave as chief executive of the Coors Brewing Co. his family founded, has no connection to the ad. He and Salazar have both criticized outside groups' involvement in the race to succeed retiring GOP Sen. Ben Nighthorse Campbell (search).

Floyd Ciruli, an independent political consultant in Denver, said he expects more campaigning from outside interests because of the race's national importance.

While the war in Iraq, health care and experience emerged as issues even before the primaries, the ads have given the environment an unusually prominent role in the campaign. Coors could face questions about the brewery's environmental record, Ciruli said.

Indeed, the League of Conservation Voters will spend the fall discussing Coors Brewing's environmental issues, said Andy Schultheiss, the group's regional director.

"It will be mostly door to door, but you may see some TV ads later," Schultheiss said Wednesday.

The Golden-based brewer agreed in 2001 to build a wetland to settle a lawsuit by the state for an accidental discharge of beer that killed more than 50,000 fish in a creek. It also paid a $117,280 fine. A similar accident killed about 13,000 fish a decade earlier.

Coors campaign spokeswoman Cinamon Watson said Coors, a national past president of Ducks Unlimited, has an excellent record on conservation and is looking forward to discussing it.

On the mine pollution issue, Salazar's ad counters that he took over the natural resources job in 1990, well after problems at the gold mine erupted. And the ad from the League of Conservation Voters praised Salazar for crafting one of the largest environmental settlements in the state's history and creating an environmental crimes unit in the attorney general's office.