David Goldman/AP

He has called Mitt Romney a "liar" who "misled the entire country" in last week's debate. But that won't keep Newt Gingrich from endorsing Romney should he become the Republican nominee for president.   "I think reelecting Obama is a disaster and I'll certainly endorse the Republican nominee. But I think that Mitt Romney will have a very, very hard time differentiating himself [from Obama]," Gingrich told me this morning on "GMA."   Gingrich, who only a week ago was riding high from his South Carolina victory, is now plunging in the polls. T he front page of USA Today shows Gingrich trailing Obama by 14% while Romney is in a statistical tie with the president.   But in the 24 hours before Floridians hit the polls Gingrich is making his case against nominating a man who he thinks is too moderate for the electorate.   "We nominated a moderate in 1996, he got beaten badly. We nominated a moderate in 2008, he got beaten badly. I know that the establishment would love to nominate Romney and they want to make him electable so they can beat [Obama] in the Fall," he said. "But I think the record of '96 and 2008 is you better have a conservative who can draw a really wide gap."   Gingrich said he's the man to do it and pointed to what billionaire George Soros told Reuters last week that "either you'll have an extremist conservative, be it Gingrich or Santorum, in which case I think it will make a big difference which of the two comes in. If it's between Obama and Romney, there isn't all that much difference except for the crowd that they bring with them."   "There are a lot of parallels between these two guys. Romneycare and Obamacare are essentially the same. He can bury me for a very short amount of time with four or five or six times as much money, most of it raised on Wall Street from the guys who got bailouts from the government," Gingrich told me. "In the long run the Republican Party is not going to nominate the founder of Romneycare, a liberal Republican who is pro-abortion, pro-gun control and pro-tax increases."   "Ain't gonna happen," he added

- George Stephanopoulos