Newt Gingrich is calling Republican presidential campaign rival Mitt Romney "a Nelson Rockefeller Republican" who will have a hard time appealing to the GOP's base.

Gingrich also calls himself "a very complicated candidate," telling CNN's "State of the Union" he has to get people to embrace his "campaign of substance."

Of Romney, Gingrich calls him "a very likable person" who works hard. But Gingrich says Romney's challenge is, "People look and they say, `Not yet.' "

He also says he initially saw Texas Gov. Rick Perry as a "natural alternative" to Romney but that Perry has stumbled. Gingrich spoke highly of fellow Georgian Herman Cain, saying he has a good chance to be the nominee "if he can explain a 9 percent sales tax so that people will want it."