San Francisco ? A fierce battle is brewing over the future of the Sierra Club, and an unlikely issue is at the center of the debate: immigration.

A growing faction in the nation’s most influential environmental group has urged a stronger stance against immigration, calling the growing U.S. population and its consumption of natural resources the biggest threat to the environment.

Past and present Sierra Club leaders say the anti-immigrant faction has teamed up with animal-rights advocates in an attempt to hijack the 112-year-old organization and its $100 million annual budget.

“At stake is really the heart and soul of the organization,” said Adam Werbach, the club’s president from 1996-98. “It’s a sad attempt by a very small special-interest group to take over the entire Sierra Club organization.”

Some of the old guard has organized a movement called Groundswell Sierra to oppose what they say is an attempted takeover by outside groups. Their opponents responded by filing a lawsuit claiming the leaders are unfairly trying to influence an upcoming board election.

Between March 1 and April 15, members will cast mail-in ballots to fill five open seats on the club’s 15-member governing board. The club’s anti-immigration faction says it needs only three more seats to control policy.

“It’s a democratic process. To accuse these candidates of taking over the Sierra Club is like accusing the Democrats of taking over the White House,” said board member Paul Watson, who co-founded Greenpeace and now heads the Sea Shepherd Conservation Society.

Groundswell Sierra was formed after club leaders learned that Watson, who won a seat as a petition candidate last year, spoke openly about a takeover attempt during a speech at a conference on animal rights. Animal-rights advocates want the club to denounce hunting, fishing and meat consumption.

Club leaders say the anti-immigration debate has drawn in outsiders who want to promote their agenda. The Southern Poverty Law Center, a Montgomery, Ala.-based civil liberties group, has reported that extremist racist and anti-immigration groups are encouraging their members to pay $25 to join the Sierra Club and vote in the election.

Groundswell Sierra is encouraging members to vote because less than 10 percent of the club’s 750,000 members have participated in recent elections, making it easy to sway the vote.

The Sierra Club, the country’s oldest and largest environmental group, has traditionally advocated for clean air and water and protection of wildlands and wildlife.