Phoenix ? With millions of dollars at stake, the federal government will try to convince the Supreme Court on Monday that it took good care of tribal lands — but owes nothing even if it didn’t.

Two cases involving the Navajo and White Mountain Apache tribes, scheduled for back-to-back oral arguments, could ultimately force the Interior Department to take financial responsibility in its role as a trustee over all reservation lands and resources.

The Navajo stand to gain up to $600 million ” a major windfall even for a reservation that, as the country’s largest, sprawls across Arizona, New Mexico and Utah. Many homes lack indoor plumbing or electricity, and 40 percent of families live in poverty, according to the Census.

“If the opinion is favorable to the Navajo Nation, it means that we can catch up with a lot of the projects that have been put on hold,” said Ray Baldwin Louis, a spokesman for tribe President Kelsey Begaye.

A Department of Justice spokeswoman wouldn’t comment on the cases.

The high court must decide whether the government has the same obligations ” and potential financial liabilities ” as other trustees to act in the trust’s best interests, or if the government serves a more administrative role for the sovereign nations.

The Bush administration is arguing that the government can’t be held financially responsible in the cases because Congress didn’t specifically open itself to such claims.

Appeals courts sided with the two tribes, however, saying the Navajos may deserve up to $600 million for lost coal mining royalties and the White Mountain Apaches $14 million for repairs to historic buildings.

Even so, tribes worry that the final rulings will create a “sort of close-enough-for-government-work standard” for the federal government, said Robert Anderson, a University of Washington professor who specializes in Indian law.

On the other hand, a Supreme Court ruling favoring the two tribes might encourage others to jump into the legal ring. The Hopis already have filed a suit similar to the Navajo mining case, which dates to 1984.