In the heart of Tucson's South Side, a new billboard overlooks the mostly Hispanic area as it fans the flames of the illegal-immigration debate.

"STOP the INVASION," the bold letters in red, white and blue scream to drivers headed west on Interstate 10. "Secure our borders," it demands in smaller letters. A few dark splotches of paint — likely applied by opponents of the message — already mar the sign.

The billboard is one of 12 that have gone up in several states, including California, Texas and Florida. The Grassfire.org signs are part of a national campaign that seeks to pressure lawmakers to clamp down on illegal immigration.

"It's like right out there, in your face," Alex Arenas, 25, said of the billboard tucked in the rear of Southgate Shopping Center at South Sixth Avenue and Interstate 10. "It's kind of rude, especially over here."

As Arenas and his friend, Jesus Figueroa, 26, worked out at a gym near the billboard, Grassfire.org President Steve Elliott and 14 Tucsonans who like the billboard's message huddled by a U.S. flag for a news conference. Elliott, the only speaker, said the campaign was launched in May and "represents literally millions of citizens across this country."

The United States is being overrun with people who come here illegally, Elliott said, which "truly amounts to an invasion of our community."...