House Budget Committee ranking member Chris Van Hollen (D-Md.) speaks during a news conference on Capitol Hill in July. (Mark Wilson/Getty Images)

The Sierra Club has endorsed Rep. Chris Van Hollen in Maryland’s U.S. Senate race, weighing in on a contentious Democratic primary .

Van Hollen is competing against Rep. Donna Edwards for a seat being vacated by retiring Sen. Barbara A. Mikulski (D).

“Chris is a climate champion, an environmental leader and a clean energy jobs creator,” Sierra Club President Aaron Mair said at a press conference on the shore of the Anacostia River in Bladensburg. “He has shown that he has the fortitude to craft and pass legislation that will combat the climate crisis. But more importantly, he will continue to do so when he’s in the United States Senate.”

Baird Straughan, who leads the Sierra Club in Maryland, added that Van Hollen “has proven his ability to work through the gridlock of the U.S. Senate.”

Van Hollen and Edwards have similar records and positions on this issue; both score highly with environmental groups. Edwards previously served on the board of the League of Conservation Voters and worked on legislation to fund the Chesapeake Bay Environmental Restoration Program.

Van Hollen has pushed for a system that would cap carbon emissions while distributing dividends from excess pollution to citizens. As a state legislator, he successfully blocked oil drilling in the Chesapeake Bay — a victory he has touted in his campaign ads this year. Both oppose the Trans-Pacific Partnership, which the Sierra Club and other environmental groups have lobbied against.

However, Van Hollen has supported several free trade agreements that Edwards opposed. Bonnie Bick, political chair of the Sierra Club’s southern Maryland arm, said the group should have endorsed Edwards.

“Maryland needs a Senator who will stand up to the special interest polluters every day,” she said in a statement.

Years before Edwards ran for Congress in 2008, she and Bick fought the National Harbor development in Prince George’s County.

Patuxent Riverkeeper Fred Tutman, a Sierra Club member, likewise said Edwards “has been a stalwart for environmental protection for as long as I can recall.”

While Edwards has positioned herself as the more progressive candidate in the race, Van Hollen has argued that he is more effective and more likely to achieve progressive goals. He has been rounding up support from liberal politicians and groups. The Service Employees International Union has endorsed him, as has former gubernatorial candidate Heather Mizeur. Several prominent Maryland environmentalists endorsed Van Hollen last summer.