KYKOTSMOVI, Ariz. - An attorney told a packed gathering of Hopi that a 150-year-old document could hold the key to sovereignty and prevent the federal government from leasing their assets.

Speaking to an overflow crowd at the Hopi Veterans' Center, Indian water and land rights expert Lana Marcussen explained the 1848 Treaty of Guadalupe Hidalgo, pointing out those rights have already been upheld for New Mexico Pueblos.

Based on the treaty, Hopi may be able to claim real property rights for the land, water and minerals of their homeland. Hopi real property rights would supersede all other rights, including those claimed by the federal government.

''Because the Mexican government recognized you as individual sovereigns together in a city or pueblo, your right is superior, as is your right to control, without the permission or participation of the federal government, your land and your water,'' Marcussen told the gathering, sponsored by Black Mesa Trust.

While Hopi have not yet asserted their sovereign rights within this treaty, Pueblo Indians in New Mexico have - and have won in court.

Marcussen said the rights of Pueblo people as individuals take precedence over the rights of tribal governments imposed on the tribes by the Indian Reorganization Act of 1934.

When the U.S. government accepted the Treaty of Guadalupe Hidalgo, and specifically Article VIII, it committed to protecting all of the property of the people who chose to remain where they were in the lands ceded by Mexico. These people were Mexican citizens, and the United States promised to respect their rights whether they chose to remain Mexican citizens and relocate, or stay where they were and eventually become U.S. citizens.

Initially, in California in 1851, the U.S. government asked the Pueblo peoples to provide proof of their land grants and nearly all of the land claims were accepted. However, after losing half of its land in California, the government realized it had made a mistake.

Beginning in 1854, courts decided whether to accept the land grants in Arizona and New Mexico. Although there were huge losses of land in those states for individuals, all 23 claims submitted by the Pueblos, in what is now New Mexico, were eventually proved and honored.

Based on this treaty right, Marcussen and her colleagues have won primary water rights - not just federal reserved rights - for tribes in New Mexico.

In the treaty, signed after nine years of war between the United States and Mexico, Mexico ceded 55 percent of its territory, which is now Texas, New Mexico, Arizona, California and parts of Colorado, Nevada, and Utah. Mexico received $15 million in war reparations from the United States.

Marcussen said the profound issue is sovereignty. Under the Constitution, power is distributed between two sovereigns: the federal and state governments. Under this principle, the third sovereign is the people, as individuals and as a group; and the people's rights are superior to both federal rights and states' rights. Tribal sovereignty, not specified by the Constitution, has had to find its place within this basic structure, she said.

The Treaty of Guadalupe Hidalgo turns the theory of the United States having the ultimate power to hold Indian land in trust upside down, she said.

The Pueblo Revolt of 1680 resulted in land grants for the Pueblos. The pope declared that the Pueblo Indians, by virtue of the fact that they lived in cities and had a social and political organization sophisticated enough to drive the priests out of the Southwest at will during the revolt, were civilized people, she said.

Since the Spanish visited Hopi as early as 1540, it is likely that there is a land grant to the Hopi (called ''Moqui'' at that time). If there is, the Hopi are entitled to Article VIII rights under the Treaty of Guadalupe Hidalgo.

She said this means that when the federal government holds land or water or natural resources in trust for the Hopi, it is merely a trustee, not a sovereign. Therefore, the United States does not have the right to negotiate the sale or lease of that property or force the Hopi to accept the terms that the United States government has set with the Department of the Interior for the benefit of multinational corporate interests.

These treaty rights would establish rights in the case of corporations seeking Hopi land and water rights.

''If we can establish that you have Article VIII rights, you are the sovereign. If you are the sovereign, you can say, 'I don't care what you pay; we will not sell.'''

Marcussen said the U.S. government is still having a difficult time accepting that Indian peoples could have real property rights. However, those rights have been affirmed by the courts for the New Mexico Pueblos and for the Tohono O'odham in Arizona.