DENVER — La reconquista, a radical movement calling for Mexico to “reconquer” America’s Southwest, has stepped out of the shadows at recent immigration-reform protests nationwide as marchers held signs saying, “Uncle Sam Stole Our Land!” and waved Mexico’s flag.

Even as organizers urged marchers to display U.S. flags, the theme of reclaiming “stolen” land remained strong. One popular banner read: “If you think I’m illegal because I’m a Mexican, learn the true history because I’m in my homeland.”

“We need to change direction,” said Jose Lugo, an instructor in Ethnic Studies at the University of Colorado at Boulder at a campus march last week. “And by allowing these 50,000, 50 million [immigrants] to come in here, we can do that.”

The revolutionary tone has surprised even longtime immigration watchers such as Ira Mehlman, the Los Angeles-based spokesman for the Federation for American Immigration Reform.

“I’ve always been skeptical myself about this [reconquista], but what I’ve seen over the last few weeks leads me to believe that there’s more there than I thought,” Mr. Mehlman said.

“You’re seeing people marching with Mexican flags chanting, ‘This is our country.’ I don’t think that we can dismiss this as youthful exuberance or a bunch of hotheads,” he said.

Hispanic rights leaders insist there’s nothing to the so-called reconquista, sometimes referred to as Aztlan, the mythical ancestral homeland of the Aztecs that reportedly stretches from the border to southern Oregon and Colorado.

Nativo Lopez, president of the Mexican American Political Association in Los Angeles, one of the march organizers, was infuriated when a reporter asked him about the reconquista.

“I can’t believe you’re bothering me with questions about this. You’re not serious,” Mr. Lopez said. “I can’t believe you’re bothering with such a minuscule, fringe element that has no resonance with this populous.”

At the same time, some analysts say the seismic demographic shifts brought on by unchecked border crossings and birth rates are resulting in a de facto reconquista.

“Demographically, socially and culturally, the reconquista of the Southwest United States by Mexico is well under way,” Harvard University professor Samuel P. Huntington said in 2004.

“No other immigrant group in U.S. history has asserted or could assert a historical claim to U.S. territory. Mexicans and Mexican-Americans can and do make that claim,” he said.

A three-minute videotape made by the Immigration Watchdog Web site plays speeches by Hispanic professors and elected officials making references to Aztlan and the idea of a demographic takeover.

“We are millions. We just have to survive. We have an aging white America. They are not making babies. They are dying. It’s a matter of time. The explosion is in our population,” Jose Angel Gutierrez, political science professor at the University of Texas at Arlington, said on the videotape.

In an interview, Mr. Gutierrez said there was “no viable” reconquista movement. He blamed interest in the issue on closed-border groups and “right-wing blogs” such as American Patrol and L.A. Watchdog, but those Web sites are getting plenty of ammunition from groups like La Voz de Aztlan, a Whittier, Calif.-based news service that advocates a separatist state while criticizing Jews and “gringos.”

Then there’s the Mexica Movement, which wants to “reconstruct” the United States as an “indigenous” nation called Anahuac. Professor Charles Truxillo of the University of New Mexico envisions a sovereign Hispanic nation called the Republica del Norte that would encompass Northern Mexico, Baja California, California, Arizona, New Mexico and Texas.

MEChA, an acronym for the Chicano Student Movement of Aztlan, has come under fire for revolutionary language in its “El Plan de Aztlan,” a founding document that declares “the independence of our mestizo nation,” decries the “brutal gringo invasion,” and says that land “rightfully ours will be fought for and defended.”

What’s notable about MEChA is its otherwise mainstream image. Most Hispanic leaders, including Los Angeles Mayor Antonio Villaraigosa, belonged to MEChA in high school or college. Former Lt. Gov. Cruz Bustamante came under fire from conservatives for refusing to renounce his membership during the 2003 gubernatorial race.

Federico Rangel, a University of Colorado graduate student and MEChA officer, said most students view Aztlan as part of their history, not as a rallying cry for revolution.

“Aztlan isn’t what people say it is, like the reconquista,” said Mr. Rangel, who carried a MEChA sign at Monday’s rally. “It’s a spiritual homeland to Chicanos.”