"We're going to run an American campaign that appeals to every person in every state. We're not going to give up a single neighborhood," Gingrich said.
Gingrich also knocked Romney's support for indexing the minimum wage to inflation, which would result in gradual and automatic increases.
"If we had done that in the Carter years, we would have massive unemployment in the European tradition," Gingrich said.
The former Speaker argued that the minimum wage kept businesses from hiring more youths who would benefit from work experience.
"This is totally unacceptable," he added. "Young people who don't learn the work habits, who don't learn to budget the money they earn ... are literally trapped out of the American dream."
Romney said Wednesday to reporters on his campaign plane that he hadn't "changed my thoughts" on his 2002 campaign proposal that advocated an increase in the minimum wage and indexing it to inflation.
Romney also defended his statement about concern for the poor to reports Wednesday.
"No no no no. No no. You've got to take the whole sentence, all right, as opposed to saying, and then change it just a little bit, because then it sounds very different. I've said throughout the campaign my focus, my concern, my energy is gonna be devoted to helping middle-income people, all right? We have a safety net for the poor, and if there are holes in it, I will work to repair that. And if there are people that are falling through the cracks I want to fix that," Romney said.
"Wealthy people are doing fine. But my focus in the campaign is on middle-income people. Of course I'm concerned about all Americans — poor, wealthy, middle class, but the focus of my effort will be on middle-income families who I think have been most hurt by the Obama economy."
The Romney campaign blasted Gingrich's criticism as an attempt to distract from his record.
“It’s no surprise that Newt Gingrich would join the Democrats in distorting Mitt Romney’s comments," Romney spokeswoman Amanda Henneberg said. "The sad fact is that Newt Gingrich will do anything to distract from his work for Freddie Mac. When Nevada families were struggling in the housing crisis, Newt Gingrich was being paid $1.6 million by Freddie Mac. He was not sticking up for Nevada families – he was cashing in.”
Gingrich made no mention of the expected endorsement by reality show host Donald TrumpDonald TrumpClinton: Photos from women’s march ‘awe-inspiring’ Is McCain confident in Trump? ‘I do not know’ Yemen reports first drone strike under Trump MORE of Romney later Thursday afternoon. Gingrich had actively courted the casino mogul's support, and as recently as Wednesday evening, his campaign staffers were leaking that an endorsement was imminent. Nevertheless, it appears Trump ultimately decided to side with the presumptive front-runner in the GOP race.