The League of Conservation Voters' (LCV) political arm is endorsing Nevada Democratic Rep. Jacky Rosen's bid to topple Republican Sen. Dean Heller Dean Arthur HellerOn The Trail: Democrats plan to hammer Trump on Social Security, Medicare Lobbying World Democrats spend big to put Senate in play MORE.

Nevada is considered one of the top Democratic offensive opportunities in the country, thanks to victories last November by both Hillary Clinton Hillary Diane Rodham ClintonWhat Senate Republicans have said about election-year Supreme Court vacancies Bipartisan praise pours in after Ginsburg's death Trump carries on with rally, unaware of Ginsburg's death MORE and now-freshman Sen. Catherine Cortez Masto (D).

The early endorsement is a signal that Rosen will continue to draw major liberal power-players to the closely watched race.

“In Congress and as a community leader, Jacky Rosen has been a strong supporter of investing in clean energy and protecting Nevada’s parks and monuments,” said LCV Action Fund Senior Vice President of Government Affairs Tiernan Sittenfeld.

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“We are thrilled to back Jacky Rosen against Senator Heller, who has a terrible record of voting to undermine protections for our air and water while supporting every climate change denier in Trump’s Cabinet. With a 13 percent lifetime score, Senator Heller sides with special interest polluters almost every chance he gets.”

Rosen is the clear favorite in the Democratic primary — she faces no major opposition so far, and has a smattering of key endorsements in the state. But Rep. Dina Titus (D-Nev.) hasn't ruled out a bid of her own.

Despite the potential of a Titus bid, a significant number of liberal allies have already thrown their weight behind Rosen. Former Senate Democratic Leader Harry Reid Harry Mason ReidSenate Republicans signal openness to working with Biden Mellman: The likely voter sham Bottom line MORE (D-Nev.), the Democratic Senatorial Campaign Committee, Cortez Masto and NARAL Pro-Choice America have all backed Rosen's bid.

Nevada's leftward trend has made the state one of the few where Democrats could pick up a Senate seat in 2018. The party has to defend 23 seats next year, including 10 in states that President Trump won in November.

But Nevada was one of the few bright spots for Democrats last November. Clinton won the presidential vote there by 2 points, while Cortez Masto won her marquee Senate race by 3 points.

Rosen has only been in Congress since January, but the endorsement is a sign of her previous legislative record. The LCV applauds her stance against reopening the Yucca Mountain nuclear waste facility and cites her support for other pro-environment legislation as well.

The LCV gave Heller a low rating on its annual scorecard, with a lifetime score of 13 percent. The group takes issue with his vote against the Clean Water Rule as well as a recent vote to quash the Methane and Waste Rule earlier this year.

Heller's official website points to a handful of environmental accomplishments, including backing tax credits for creating solar energy and legislative language to protect the Pine Forest Range Wilderness Area.

The LCV’s support for Rosen could turn into financial gain, as the group will help to direct supporters to donate to her campaign. The LCV helped channel six-figure donations to 10 separate Senate candidates last year, including $500,000 toward Cortez Masto's bid.