TUCSON, Ariz. – If the government doesn’t build security fencing along the Mexico border, Minuteman border watch leader Chris Simcox says he and his supporters will.

Mr. Simcox, whose civilian watch group opposes illegal immigration, said Wednesday he was sending an ultimatum to President Bush to deploy military reserves to the Arizona border by May 25 or his supporters will break ground for their own building project.

“We’re going to show the federal government how easy it is to build these security fences, how inexpensively they can be built when built by private people and free enterprise,” Mr. Simcox said.

Congress has been debating immigration reform for several months. One bill, approved by the House in December, calls for nearly 700 miles of fencing along the American-Mexican border. The fence proposal has angered Mexicans, with President Vicente Fox calling it “shameful.”

Mr. Simcox said a half-dozen landowners along the Arizona-Mexico border have said they will allow fencing to be placed on their borderlands, and others in California, Texas, and New Mexico have agreed to do so as well.

Surveyors and contractors have offered to help with the design and survey work, and Mr. Simcox said some have said they will provide heavy equipment for his Minuteman Civil Defense Corps. to build fencing.