Giuliani told small-business owners he would not punish them for unwittingly hiring illegal immigrants.
Federal officials are "trying to put the responsibility for this on employers, on city government, on state government," the former New York mayor said during a conference call arranged by the National Federation of Independent Businesses.
"The simple fact is, nobody but the federal government can stop people from coming into this country illegally, and the federal government does a very bad job of that," Giuliani said.
He said no other presidential candidate will solve the problem.
"If you elect a Democrat, they're just going to open the borders, and more illegals are going to come in," he said.
"And if you elect one of my (Republican) opponents, they want to crack down on cities and states, and they want to crack down on businesses, but they don't want to solve the problem," he said. "If I become president, in a very short while, you will not be able to walk into the United States without identifying yourselves."
Giuliani says he would build a fence along the U.S.-Mexico border that includes high-tech monitoring to detect those trying to enter the U.S. illegally. He also calls for hiring more border patrol agents.
Legal immigrants should be issued a tamper-proof, federal identification card, he said, "and if something is wrong with that card, it's the federal government's responsibility, not yours."
Republican candidates have been sparring over illegal immigration, an issue that angers conservatives who hold sway in party primaries and who argue that the problem is straining schools and hospitals and taking jobs away from U.S. citizens.
Former Massachusetts Gov.has criticized Giuliani for defending New York's "sanctuary" policy, which barred city workers from reporting suspected illegal immigrants who enrolled their children in school or sought hospital treatment.
"If the mayor is just looking to place blame, all he needs to do is look in a mirror," Romney spokesman Kevin Madden said Tuesday. "Stopping illegal immigration is about actually enforcing existing immigration laws."
Of the sanctuary city criticism, Giuliani has responded that, while he cracked down on all lawlessness, Romney tolerated sanctuary cities in Massachusetts.
Romney argues he tried to curtail the problem by deputizing state police to help enforce federal immigration laws.
Former Tennessee Sen., has accused both Giuliani and Romney of being weak on immigration.
"As mayor, Rudy Giuliani told city employees to ignore federal law and barred them from assisting federal immigration agents, and yet he now expects voters to believe he'd be tough on our borders?" said Thompson spokesman Jeff Sadosky.
Also Tuesday, Giuliani criticized President Bush's No Child Left Behind education law, saying, it's "really a debate about what kind of standards do federal governments impose on state and local bureaucracies."
"I would turn the decision-making over to people and to parents," he said on the conference call. "I'm willing to bet that parents will do a better job of evaluating these schools that are doing a poor job a lot better than government bureaucrats do."