WASHINGTON (AFP) – The political tussle over US spending has ensnared an unlikely victim, the gray wolf, whose long-time status as an endangered species will likely be axed due to a late addition to the budget deal.

The annex, or rider, attached by two senators to the federal budget bill after weeks of tumultuous debate, marks the first time that Congress has removed an animal from the endangered species list and is expected to pass in a vote on Thursday.

Added Tuesday, a few days after a deal to prevent the government from shutting down was agreed on, the move has left environmentalists both seething and admitting defeat after years of legal wrangling over the fate of the wolves.

"There is nothing we can do to sue because the rider actually bans the citizens from suing the government over this issue," said Kieran Suckling, executive director of the Center for Biological Diversity.

"We are going to have to regroup at this point and come at wolf recovery from a fresh angle because we have been shut down," he told AFP.

At issue is whether wolves, which were heavily hunted in the US west for many decades, have recovered in numbers enough to allow hunters to target them again.

The wolves had all but disappeared from the region until they were reintroduced in the 1990s, and their protected status has allowed them to reach a population of 1,651 in the Rocky Mountain region, according to the Sierra Club.

But ranchers say wolves are a nuisance to livestock and could even threaten humans if their population grows too large.

The number of 300 wolves was decided upon as a regional threshold in the late 1980s, even before efforts began to re-establish a wolf population, said Sierra Club spokesman Matt Kirby.

"It was an arbitrary number. It was not based on any science. It was picked out of the air," he told AFP.

Since then, "the science has gone a lot farther and shown that 300 is not enough to have a genetically connected population and to really have a sustainable population, which is the intent of the Endangered Species Act," Kirby said.

The rider caps a legal battle that dates back to the end of the George W. Bush administration, and allows the removal of the gray wolves from the list maintained by the Fish and Wildlife Service under the 1973 Endangered Species Act.

The Bush Administration set the delisting in motion during its final weeks in power. The controversial move was upheld by the Barack Obama administration, but 14 environmental groups sued and won their case to prevent it from happening in 2010.

Tuesday's rider reverses that and effectively puts an end to the matter by preventing further legal action.

Two senators, Republican Mike Simpson of Idaho and Democrat Jon Tester of Montana, both from states with growing wolf populations, added the rider to the compromise bill -- agreed shortly before midnight on Friday (0400 GMT Saturday) -- that funds the US government to October 1.

Tester, who chairs the Congressional Sportsmen's Caucus, said in a statement that the "bipartisan provision" would return wolf management to the states and remove protected status because the once-vulnerable population has recovered.

"Right now, Montana's wolf population is out of balance and this provision will get us back on the responsible path with state management. Wolves have recovered in the Northern Rockies," he said.

"By untying the hands of the Montana biologists who know how to keep the proper balance, we will restore healthy wildlife populations and we will protect livestock."

Environmentalists allege that Tester is facing a perilous re-election bid in a remote, right-leaning state where hunting is popular, and is seeking to gain favor from voters.

"While normally the Democratic party and the White House would oppose this and not let a bill go through, they have decided it is more important to boost Jon Tester's poll numbers for the upcoming election than to protect wolves," said Suckling.

"It doesn't save any money at all. It is a terrible harm to the economy," he added. "The reintroduction of wolves into Yellowstone National Park and the Rocky Mountains have been a huge tourist draw."

Kirby accused Congress of meddling with a federal act that should not be decided on a state-by-state basis.

"The concern is once this has happened, you have really opened the door to politicians cherry-picking the individual species that are inconvenient and just introducing legislation to do away with them," Kirby said.

"We really have to work to make sure Congress does not do this again."