Bringing an outside candidate like Mitt Romney into the presidential race would be a huge mistake for Republicans, as it would be the same as handing the election to Hillary Clinton, former New York Mayor Rudy Giuliani said Monday."To me, that would be a disaster," Giuliani told Fox News' "Fox & Friends" program. "I can't see how you could be a Republican and want Hillary Clinton to be president of the United States."Instead, the party spoke and made the choice, said Giuliani, and now Republicans should unite behind Trump as the eventual nominee.Meanwhile, House Speaker Paul Ryan has said he has some hesitation about supporting Trump, but Giuliani said that on the main issues core Republicans hold near, including lower taxes, lower regulations, making America stronger, and fixing the domestic economy, "Donald Trump is there," said the former mayor, who endorses the New York real estate mogul."On the main issues of the Republican Party, he's going to be much better for this country than Hillary Clinton who's going to raise taxes," said the former mayor. "I read somewhere yesterday that more regulations have been written in this administration than the last 10 administrations combined."She's going to write twice as many. We're going to go from Obamacare to Hillarycare. Single payer, socialized medicine in this country. Those are big, big differences. Plus, his approach to the world is a much more muscular approach, much more support for the military."Trump will meet with Ryan on Thursday, and Giuliani said that he thinks everyone should hold back on their attacks until after that happens, including former Alaska Gov. Sarah Palin , who vowed over the weekend to help Ryan's primary opponent to win the Wisconsin House seat."We should stop fighting amongst ourselves," said Giuliani. "Sarah should pull back . . . Paul Ryan is an imminently reasonable man and Donald Trump and Paul Ryan have no personal history."The men do have some policy disagreements, Giuliani admitted, but he said he also doesn't always agree eye-to-eye with other Republicans on policy himself.Further, he said he doesn't agree with Palin that Ryan's career is over, and he thinks Palin isn't interpreting Ryan's intentions correctly."I think I know Paul better than Sarah does," said Giuliani. "If he were running, he would have run this time. I think Paul is one of these guys in politics, I wish we had more of them, who's very principled. This is what he believes in."