Former New York City Mayor Rudy Giuliani slammed Mitt Romney as an unelectable flip-flopper, and said Newt Gingrich, whom he compared to Ronald Reagan, offers Republicans the best shot at unseating President Obama.

Speaking Thursday on MSNBC’s "Morning Joe," Giuliani recalled his GOP candidacy in 2008, in which he ran against Romney.

“I’ve never seen a guy change his positions on so many things, so fast, on a dime, on everything,” Giuliani said. “Pro-choice, pro-life. And pro-choice because somebody, a close friend, died, and he became pro-choice because this woman died of an abortion. Then he figures out there are embryos and he changes.

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“Then he was pro-gun control,” Giuliani continued. “Fine. Then he becomes a lifetime member of the NRA. Then he was pro-cap-and-trade. Now he’s against cap-and-trade. He was pro-mandate for the whole country, then he becomes anti-mandate and he takes that page out of his book and republishes the book. I could go on and on.”

Giuliani said this opens Romney to an attack from President Obama in the general election that “this is a man without a core,” “a man without substance” and “a man that will say anything to become president of the United States.”

Although he hasn’t made an official endorsement, Giuliani, for the second time this week, went on record supporting Newt Gingrich, and even compared him to a Republican icon — former President Ronald Reagan.

“I kind of go back to 1980 and I remember the Carter White House just dying [to run against] Ronald Reagan,” Giuliani said. “Ronald Reagan was the dumb actor, Ronald Reagan said incendiary things, Ronald Reagan was like Newt — gosh, you never knew what he was going to say and the whole world would go crazy — The New York Times would write editorials. There was Bush, greatest resume of anyone who ever ran for president, solid citizen. They got Reagan and they got trounced.”

Giuliani said that although Gingrich has a long record in Washington that sometimes conflicts with conservative principles, so too did Reagan.

“I think Newt has a much more consistent position as a conservative, with some real exceptions like Ronald Reagan had,” Giuliani said, comparing Gingrich to Romney. “Ronald Reagan signed a bill that made abortion legal in the state of California. Ronald Reagan did in fact raise taxes several times, not just as president, but also as governor of California.”

Giuliani also said that Gingrich is the more electable candidate — an argument that Romney supporters have pushed for their candidate even as Gingrich has risen in the polls.

“It may be that Newt is appealing to some that maybe Mitt isn’t appealing to,” Giuliani said. “There’s something wrong when you’ve been running as long as Mitt has and you’re at 25 percent, and you don’t go much below, and you don’t go much above. Seventy-five percent of the other Republicans are telling you something.”

Romney has sharpened his attack against Gingrich in recent days, going after him as a “zany” lifelong politician. Gingrich has vowed to adhere to the Reagan principle of not speaking critically against other Republican, but will likely rely on surrogates to get that message out.