Red, White and Blue Fund, which backs Santorum, was hardly kinder. It aired commercials saying that Romney and Obama "aren't much different" on key issues such as federal spending and supporting a requirement for individuals to purchase health care coverage. That's a reference to a Massachusetts law that Romney signed in his home state that bears similarities to the legislation Obama won from Congress.
Gingrich, who said over the weekend that Romney is the weakest Republican front-runner in nearly a century, is backed by a super PAC airing ads, as well.
His campaign is advertising at lower levels, including a commercial that hits Obama for high gas prices and another that says the president modeled his health care reform plan after the one Romney had enacted in Massachusetts.
The former House speaker also using a recorded phone message from Chuck Norris in Alabama.
"As president, Newt will repeal Obamacare, get rid of Obama's czars, and use commonsense measures, like building the Keystone Pipeline to lower the cost of gas to two and half dollars a gallon," says Norris, whose website notes he is a movie star and World Professional Middle Weight Karate Champion.
Santorum and Gingrich employed different approaches as they campaigned during the day, the former Pennsylvania senator more critical of Romney, while the former House speaker focused his attacks on Obama.
Santorum said his two rivals have changed positions on the issue of global warming.
"I didn't change as the climate changed. I stood tall. Now the climate has changed and everyone's for drilling now ... but understand that when times were tough, they were not and I was," he said.
Gingrich, at the same conference, said Obama is presiding over a "very anti-fossil fuel administration. The left wing environmental movement hates oil."
Romney made the economy his text for criticizing Obama.
He said the president wrongly thinks the country is doing better because of recent increases in employment. More than 200,000 jobs have been created in each of the past three months, but Romney said the president, "should go out and talk to the 24 million Americans who are out of work or stopped looking for work or are unemployed."
Associated Press writers Charles Babington, Beth Fouhy, Philip Elliott and Phillip Rawls in Alabama and Stephen Ohlemacher, Kasie Hunt and Jack Gillum in Washington contributed to this story.
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