WASHINGTON (CNN) -- Republican presidential candidate Mitt Romney Tuesday stepped up his ongoing attack on cities with what he calls sanctuary policies for illegal immigrants -- including rival Rudy Giuliani's home city -- with a new radio ad set to air in crucial campaign states Iowa and New Hampshire.
Republican presidential candidate Mitt Romney campaigns in Cedar Rapids, Iowa, last week.
"Immigration laws don't work if they're ignored," the ad's announcer states. "That's the problem with cities like Newark, San Francisco and New York City that adopt sanctuary policies. Sanctuary cities become magnets that encourage illegal immigration and undermine secure borders."
"Legal immigration is great," Romney, a former governor of Massachusetts, himself says in the ad. "But illegal immigration, that we've got to end. And amnesty is not the way to do it."
He also advocates in the ad cutting back on federal funds to cities that adopt sanctuary policies. Listen to Romney's radio ad »
The two GOP front-runners have been locked in a war of words over the issue after Romney claimed Giuliani supported illegal immigration while mayor. Giuliani has repeatedly denied the charge, arguing instead that he cracked down on illegal immigrants while in office.
A Giuliani aide fired back at Romney Tuesday, criticizing him for hiring a landscaping firm that employed illegal immigrants to work at his home.
"Gov. Romney's ad conveniently forgets that his plan to deputize state troopers that never went into effect did absolutely nothing to cut down on the number of illegals working in his yard," a Giuliani aide said. "Let's hope his plan to secure the borders is more effective than that."
And last week at a campaign event in New Hampshire, Giuliani said "I took a city that had just about the highest illegality rates in the country and took it down to one of the lowest."
Giuliani also began running his own radio ad on the subject Thursday, advocating a fence along the U.S.-Mexico border.
New York has never officially declared itself a sanctuary city, but it adopted a policy in 1988 that allows people to report crimes or check in to a hospital without their immigration status being questioned. Giuliani has pointed out that three cities in Massachusetts also carried that policy while Romney headed the state.
Responding to Romney's ad Tuesday, current New York City Mayor Michael Bloomberg said, "New York's strength has always been and always will be its constant flux of immigrants." Watch Bloomberg call immigrants vital to his city »
"We welcome immigrants, want more of them, and without them this city would not have a future and I believe this country wouldn't have a future," he added.
Bloomberg has repeatedly denied speculation he is considering an independent White House bid.
Kevin Madden, a Romney spokesman, noted that Bloomberg did not make a distinction between legal and illegal immigration. Romney supports legal immigration, Madden said. E-mail to a friend
CNN's Mark Preston contributed to this report.