When the Mashpee Wampanoag tribe broke ground on a $1 billion casino last week, it surged ahead of rival developers in the race to open the state’s first full-scale casino.

The move was a calculated risk. With a federal lawsuit hanging over the Taunton project, a legal challenge backed by a competing casino, the tribe could be forced to halt construction or even shut down after opening.

But most legal observers say the tribe is on relatively safe ground: They are not aware of any casinos that have been forced to halt construction, and the tribe has the weight of the US government and its lawyers behind it.

Tribal leaders express confidence they will prevail in court.

“The odds are very slim to virtually nonexistent’’ that the lawsuit will derail the project, said Arlinda Locklear, the lawyer for the 3,000-member Mashpee.

Filed in January by a group of Taunton property owners, the lawsuit contends that the federal government acted unlawfully last year when it approved the tribe’s request to designate 150 acres as a sovereign reservation, clearing the way for a casino.

Lawyers for the Taunton residents have seized on a 2009 US Supreme Court ruling, known as the Carcieri decision, that seemed to restrict eligibility for reservations to those tribes that had an official relationship with the federal government in 1934, when the Indian Reorganization Act was enacted.

That law, sometimes known as the “Indian New Deal,’’ authorized the government to acquire property “for the purpose of providing land for Indians.’’

The lawsuit asserts the tribe’s relationship with the government did not officially begin until 2007, long after the Carcieri decision's cutoff date. It contends the Department of the Interior, which includes the Bureau of Indian Affairs, glossed over that “inconvenient truth’’ in its rush to approve the Mashpee reservation.

“The federal government grossly overstepped its authority,’’ said Adam Bond, a lawyer representing the Taunton residents, citing a “plain reading’’ of the Carcieri case.

“We are prepared to take our case all the way’’ to the Supreme Court if necessary, he said.

But legal specialists familiar with lawsuits against Indian casinos say Carcieri’s apparent 1934 cutoff date is far from ironclad.

The Interior Department, after all, saw fit to sidestep the Carcieri ruling when granting the Mashpee a reservation, and it did the same with the Cowlitz tribe of Washington state, which wasn’t recognized until 2002. That case remains under appeal, with a decision expected this summer.

Despite the lawsuit, the Cowlitz have moved forward with construction for a $500 million casino, pouring a concrete foundation and erecting a steel frame.

At issue in such legal challenges is the proper reading of the Indian Reorganization Act, which stated that only tribes “now’’ “under federal jurisdiction’’ are eligible to have lands held in trust.

But the Carcieri decision didn’t specify what “federal jurisdiction’’ meant, a failure that has drawn significant criticism, and the Interior Department has taken a broad interpretation.

In its decision to designate Mashpee lands as a reservation, the government pointed to evidence that lawmakers intended the 1934 law to be flexible, with a goal to “rebuild tribal land bases and empower tribal self-determination.’’

The government also noted that the tribe’s Cape Cod home had long been recognized as a state reservation, and said that contemporary tribe members were Indians as descendants of those who once lived there.

Robert T. Anderson, a law professor at the University of Washington, said the government’s approval of the Mashpee reservation was “a carefully reasoned decision’’ derived from “an exhaustive analysis of the Mashpee history.’’

The decision, he said, “should receive substantial deference’’ from the courts.

Dennis Whittlesey, a Washington D.C., lawyer with more than 40 years’ experience with Indian law, said the Mashpee are acting reasonably in proceeding with the casino, despite the lawsuit.

“Those challenging the Mashpee have to prove a negative — that the tribe was not under federal jurisdiction in 1934 — and it’s very hard to prove a negative in the law,’’ he said. “There’s a lot of ways to argue the tribe meets the requirements of the law, despite that Supreme Court case. And the tribe only needs one of them to work.’’

Yet the Carcieri decision, which has not been directly challenged since it was handed down, casts a long shadow. Groups such as the National Congress of American Indians, which calls the decision “a direct attack on tribal sovereignty,’’ call for legislation to clarify the situation once and for all.

For the Mashpee, protracted court battles are nothing new. In 2007, they won federal recognition as a sovereign Indian nation after a 32-year legal struggle, and quickly sought reservation status.

But two years later, the Supreme Court issued the Carcieri decision, named for the Rhode Island governor, Donald Carcieri, who challenged the Interior Department’s approval of a reservation for the Narragansett of Rhode Island.

As the legal skirmish continues, the Mashpee’s decision to begin construction may well prove pivotal.

The Massachusetts Gaming Commission is expected to decide in the coming weeks whether to approve plans for a resort casino in Brockton, less than 20 miles from Taunton, or whether the Mashpee should have exclusive rights to the region.

Neil Bluhm, the multimillionaire developer behind the Brockton plan, is partially funding the lawsuit against the Mashpee in a clear attempt to scuttle, or at least delay, a competitor’s plan.

But if the commission denies Bluhm’s bid, he might abandon the lawsuit and search for opportunities elsewhere. Without Bluhm, the Taunton property owners who filed the lawsuit would have to search for new financial backing, hardly a sure bet.

Sean P. Murphy can be reached at smurphy@globe.com. Follow him on Twitter @spmurphyboston.