Defenders of Wildlife Action Fund, an independent environmental organization, is continuing to ratchet up its offensive against Republican Rep. Marilyn Musgrave (R-Ft. Morgan), unveiling a television advertisement today hitting Musgrave for her votes against raising vehicle fuel efficiency standards.

“Marilyn Musgrave’s votes on vehicle fuel efficiency standards cost her constituents money,” said Defenders of Wildlife Action Fund President Rodger Schlickeisen. “Gas is over $4 a gallon, and yet she also recently voted against measures to prosecute gasoline price gougers.”

The organization has purchased an extended series of ad buys in the Fort Collins-area district which total in the six-figures. The spot is set to air over a span of three weeks.

It’s the third ad the organization has launched targeting Musgrave, who is widely seen as one of the GOP’s most endangered incumbents this cycle. In June the organization aired spots highlighting Musgrave’s record on fuel efficiency standards and on her contributions from big oil.

Defenders, which played a key role in ousting former Rep. Richard Pombo (R-Calif.) in 2006, has turned Musgrave into a top target this cycle. This summer, Defenders dispatched Ed Yoon, who ran the organizations effort to unseat Pombo, to run field operations in its Musgrave campaign.

In an election cycle where energy issues are earning high billing, Defenders has been active. In April it was among several environmental organizations, including the Sierra Club and League of Conservation Voters, who announced the start of a coordinated campaign to target competitive Senate races in New Hampshire, New Mexico, and Colorado. While Defenders has been active in Colorado’s 4th Congressional District, League of Conservation Voters has been focusing on the state’s Senate contest.

Republicans were quick to push back on the ad.

"The people of Colorado will see right through this ad, considering that Defenders of Wildlife opposes increased domestic energy exploration, something which an overwhelming number of Americans support," said Julie Shutley, a spokeswoman for the National Republican Congressional Committee.

Musgrave’s mainly rural district in eastern and northern Colorado, which includes the cities of Fort Collins, Loveland and Greeley, is largely conservative. But the district also has an independent streak and the environment is a top concern.

Musgrave, a third-term incumbent, is facing off against Democrat Betsy Markey in November’s general election. The nonpartisan Cook Political Report currently classifies the race in the “Tossup” category.