The D.C. Chapter of the Sierra Club announced Wednesday it is endorsing Republican Patrick Mara in the April 23 special election for an at-large seat on the D.C. Council.

In a statement, the liberal-leaning environmental organization said Mara agrees with it “on every major environmental issue facing the District.” It also noted Mara’s past employment as a staffer for the late senator John H. Chafee, a liberal Republican from Rhode Island who chaired the Senate Committee on the Environment and Public Works.

Two-thirds of the Sierra Club Political Committee and Executive Board had to vote in agreement before the endorsement of Mara was issued.

The endorsement could prove to be a significant boost for Mara’s campaign as he seeks to combat six opponents who argue voters should be skeptical of electing a Republican. Mara can now say he’s been endorsed by both the D.C. Chamber of Commerce and the Sierra Club, two groups often at the opposite ends of the political debate.

“I want to help District residents and our government have less impact on the environment,” Mara said in a statement. “We must also find innovative ways to utilize cleaner and renewable energy.”

But, early in the day, the environmentally friendly Greater Greater Washington blog announced its support for one of Mara’s opponents, Democratic Elissa Silverman.

In the editorial, the blog called on “urbanists, progressives” and “reformers” to rally behind Silverman, a former reporter.

“Ms. Silverman embraces transit, mixed-use zoning, and the need especially to safeguard pedestrians now that the city is more walkable every year,” the editorial stated. “She emphasizes the need to encourage more housing units for families as many of the young people who have moved to the District begin families and want to remain in the District’s walkable, bikeable and transit-oriented neighborhoods.”

The Web site, which focuses on smart growth and planning, said Mara hasn’t shown “depth” on the issues.