The League of Conservation Voters, which has been playing an increasingly significant role in the 2012 congressional election, announced Monday that it will spend at least $450,000 on television ads opposing Rep. Jeff Flake’s (R-Ariz.) Senate bid.

The ads focus on how Flake — who is locked in a close race with former Surgeon General Richard Carmona, a Democrat, to replace retiring Arizona Sen. Jon Kyl (R) — is backing uranium mining near the Grand Canyon. In January, Interior Secretary Ken Salazar imposed a 20-year moratorium on future uranium and other hard rock mining on 1 million acres of federal land near the iconic national park.

The television ads, which note that Flake once lobbied for a Namibian uranium company, criticize an amendment he's sponsored to reverse the ban. “It’s the most dangerous kind of Washington politics,” the ad’s narrator intones. “We can’t risk Flake in the Senate.”

Flake campaign spokesman Andrew Wilder wrote in an e-mail that the congressman does not back mining in the park itself, and mining already takes place not far from the Grand Canyon.

“But it’s no surprise that liberal, out-of-state allies of Richard Carmona would seek to distort his record," Wilder wrote. “The League of Conservation Voters supports cap-and-trade, opposes building the Keystone Pipeline and is to the left of even President Obama on many issues, and so Arizonans are right to ask why they clearly see a future ally for that agenda in Richard Carmona.”

Navin Nayak, LCV’s senior vice president for campaigns said in an interview that the group decided to weigh in on the contest because “the opportunity to win a pro-environment vote in Arizona was an opportunity we couldn't pass up.”

The group has already poured nearly $8 million into the election, launching major independent expenditures in the Virginia, New Mexico, Montana, Massachusetts and Connecticut Senate races, along with five House races and the presidential race.