Asked at a town-hall meeting to characterize the mission of U.S. troops in Iraq, Giuliani said they have done an admirable job.
"Do I think the mission overall in Iraq is the correct one, I think without a doubt it is," the former New York mayor said at Insight Technologies, which makes tactical weapon lights and laser systems for the military.
"And I think the Democrats are going to change their minds about it again," Giuliani said, noting thatand voted as senators for the initial invasion in 2003.
Edwards, who is no longer in the Senate, since has apologized for his vote. Clinton has not apologized, but has said she would not have voted for the measure authorizing use of force if she had known then what she knows now.
"I think they're going to change their minds. I think the verdict of history is going to be that it was the right decision," Giuliani said.
He argued that had the U.S. not invaded Iraq, it would now be facing two dangerous countries trying to become nuclear powers - Iraq and Iran.
"Suppose Hillary Clinton and John Edwards' new position was their position back then, that it was a mistake to take him out," Giuliani said, referring to former Iraqi leader Saddam Hussein. "Wouldn't we be dealing with Saddam Hussein becoming nuclear right now? If Iran was becoming nuclear what would he be doing? Sitting there letting his arch enemy gain nuclear power over him? Or would we now be dealing with two countries seeking to become nuclear powers."
On Iran, Giuliani criticized Clinton and Sen., D-Ill., also a candidate for president, for saying they would engage in diplomatic relations with Iran. Obama has said he would be willing to meet with Iran's leader in the first year of his presidency without conditions; Clinton has said envoys below the presidential level should begin diplomatic work.
"This is the world we live in. It's not this happy, romantic-like world where we'll negotiate with this one, or we'll negotiate with that one and there will be no preconditions, and we'll invite (Iranian President Mahmoud) Ahmadinejad to the White House, we'll invite Osama (bin Laden) to the White House," Giuliani said.
"Hillary and Obama are kind of debating whether to invite them to the inauguration or the inaugural ball," he added.
Giuliani said the U.S. should be ready to negotiate, but "you can't negotiate with people unless they want to negotiate with you."
The former mayor made the rounds in New Hampshire as his campaign began airing a new radio ad in the state in which he discusses his bout with prostate cancer and promotes the $15,000 tax breaks he says he would give families to buy private health insurance.