Former NYC Mayor Rudy Giuliani has some thoughts on the GOP field, and he generously shared them with cable news audiences this morning in appearances so sharp they could be used in anti-Romney ads.

"It may be that Newt is appealing to something that…Mitt isn't appealing to," Giuliani said on MSNBC's "Morning Joe." "There's something wrong when you've been running as long as Mitt has and you're at 25%…Seventy-five percent of the other Republicans are telling you something about him…I think it's deeper than just a) 'he's kind of staid' and everything else.

"I ran against him in '07, '08, I have never seen a guy - and I've run in a lot of elections, supported a lot of people, opposed them - never seen a guy change his positions on so many things, so fast, on a dime," said Giuliani, known for having a fairly good memory when it comes to grudges and slights. On Fox News Channel's "Fox & Friends," Giuliani criticized Romney for being "so darn negative. I was really offended by his comment that Newt Gingrich is 'zany.' That isn't the kind of language that should be coming from a candidate and it reminds me of what Mitt did in '08, where he attacked me, he attacked McCain, he attacked Huckabee. Whoever was out front Mitt would attack. Sometimes personal attacks. Sometimes these ads about, you know, family and all this other stuff. So I think this could be hurting him. This is after all a Republican primary and they don't want to see a Republican savaging another Republican…"

The criticisms coming from Romney's surrogates also offend Giuliani. "Mitt's got Governor Sununu out there, he's got this one, and that one basically saying that Newt just doesn't have the right ideas but …trying to create the impression that he's crazy, zany, crazy."

Perhaps, most damagingly, Giuliani said that Romney's flip-flopping would make him easy pickings for President Obama.

"Pro-choice/pro-life. Pro-choice because somebody, a close friend, died and he became pro-choice because this woman died from an abortion. Then he figures out there are embryos and changes," Giuliani told MSNBC. "He was pro-gun control. Fine. Then he becomes a lifetime member of the NRA. 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 takes that page out of his book, and republishes the book. And I can go on and on. ..Now what will Barack Obama do to that? What Barack Obama will do to that is: 'This is a man without a core, this is a man without substance, this is a man that will say anything to become president of the United States.' I think that is a great vulnerability."

And what about Newt?

Giuliani acknowledged that Gingrich had some issues with changing his position as well. But "nothing like" the changes from Governor Romney to presidential candidate Romney. Gingrich "has a much more consistent position as a conservative, with some real exceptions, like Ronald Reagan had." Giuliani cited positions Reagan as governor had taken in favor of abortion rights and tax increases.

The former Mayor said Gingrich was better able to get the "Reagan Democrats" because "Newt knows how to talk to them….I feel more comfortable campaigning for him in the suburbs…and selling him in the suburbs, to Reagan Democrats, than I do Mitt Romney."

-Jake Tapper and Mary Bruce