With little fanfare, the League of Conservation Voters has emerged as one of the most important outside groups in several key Senate races this year.

The fact that LCV is spending heavily in several pivotal contests — including in Virginia, Massachusetts, Montana and Connecticut — runs counter to the conventional wisdom that environmental issues rank low on voters’ priority lists. But the group has spent nearly $8 million and influenced races such as the battle for New Mexico’s open Senate seat, where Democratic Rep. Martin Heinrich is leading former GOP Rep. Heather Wilson.

“From the beginning, our thought was if we came in smart and focused in terms of our resources, we could have a really big impact,” said Navin Nayak, the group’s senior vice president for campaigns, adding that the group is targeting five House races and a few states in the presidential election.

As of now, Nayak said, LCV is the third-largest independent spender on behalf of Democratic candidates in five Senate races, behind the Democratic Senate Campaign Committee and the Majority PAC, a Super PAC.

On Monday, the group announced that it would join the Majority PAC in spending $470,000 on television ads over the next week criticizing Rep. Jeff Flake’s (R-Ariz.) effort to overturn a ban on uranium mining near the Grand Canyon.

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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. Flake, who is locked in a close race with former Surgeon General Richard Carmona, a Democrat, has co-sponsored an amendment to reverse Salazar’s decision.

Flake campaign spokesman Andrew Wilder wrote in an e-mail that the congressman does not back mining in the park itself, and that 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.”

LCV has also drawn fire from Rep. Denny Rehberg (R-Mont.), who has questioned the group’s support of Democratic Sen. Jon Tester. The group, which backed Tester six years ago, has spent $2 million this year registering 27,000 Montanans to vote by mail.

In an e-mail to supporters Sunday night, Rehberg mocked Tester for his ties to a group opposed to the Keystone XL pipeline, which is popular in Montana.

“Despite being forced by pressure from Montanans to cancel his long-scheduled appearance at the very last moment—Tester recently helped the group raise money to fund its anti-Keystone XL obstructionism by lending his name to promote their annual $500-a-plate black-tie gala in New York City, where sponsorships sold for as much as $25,000,” Rehberg wrote.

National Republican Senatorial Committee spokesman Brian Walsh called LCV “a double-edged sword for many Democratic candidates,” given the group’s opposition to Keystone and support for mandatory limits on greenhouse gas emissions: “And in red states like Montana and Arizona, that’s not helpful.”

The environmental group’s political budget — which has risen from $3.5 million in 2008 to $5.5 million in 2010 and its current total of at least $8 million — still pales in comparison to major outside groups such as American Crossroads. But the group has used an array of tactics to mobilize support for the candidates it backs, including a mail advertising effort in Virginia.

In Virginia, for example, LCV has reached one in seven voters by sending between four and seven pieces of mail to more than 500,000 households in the state who will be deciding between two former Virginia governors, Republican George Allen and Democrat Tim Kaine. The mailers criticize Allen, who is trying to reclaim the Senate seat he lost six years ago, for his ties to the oil and gas industry.