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.
His comment dismissing Obama's concerns about global warming, however, oversimplified the issue. While it's true that plants need carbon dioxide to grow, the gas also is promoting global warming. "For many plants, that's going to more than offset the benefits of carbon dioxide," said David Wolfe, a professor of plant and soil ecology at Cornell University. "If you have a plant that's dead from heat stress, you can give it all the CO2 you want, but it's not going to benefit."
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, Seth Borenstein and Jack Gillum in Washington contributed to this story.
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