WASHINGTON, April 25 (Reuters) - Democratic White House contenders Barack Obama and Hillary Rodham Clinton criticized Republican Rudolph Giuliani on Wednesday for suggesting a Democratic president would put the United States at risk of another Sept. 11-type terrorist attack.

The former New York mayor, who leads the 2008 Republican White House race in national polls, said in a speech in New Hampshire on Tuesday night the United States would be safer and fight terrorism more aggressively under a Republican president.

A Democratic president, he said, would "wave the white flag on Iraq" and take the United States "back to our pre-Sept. 11 attitude of defense," the Politico.com quoted Giuliani as saying.

While the United States ultimately will prevail in the fight against terror, he said, "the question is how long will it take and how many casualties will we have? If we are on defense, we will have more losses and it will go on longer."

Giuliani's comments were similar to arguments used by President George W. Bush and other Republicans during the 2004 and 2006 elections.

Obama and Clinton said Americans had moved beyond Republican rhetoric about Sept. 11.

"Rudy Giuliani today has taken the politics of fear to a new low and I believe Americans are ready to reject those kind of politics," Obama, an Illinois senator, said in a statement.

"The threat we face is real, and deserves better than to be the punch line of another political attack," he said.

Clinton, a New York senator, said the last six years of the Bush administration have showed "political rhetoric won't do anything to quell those threats. And that America is ready for a change."

"We have to protect our country from terrorism -- it shouldn't be a Democratic fight or a Republican fight," she said in a statement.

In 2004 Bush was re-elected after questioning Democratic Sen. John Kerry's ability to fight terrorists, and Vice President Dick Cheney said a Kerry vote could lead to another terrorist attack.

The tactic was less effective during the 2006 congressional elections, when Democrats swept to power in both houses of Congress despite Republican warnings that Democratic candidates had a "pre-9/11 mind set" that would put the country at risk.

Giuliani was mayor of New York during the Sept. 11 attacks, and much of his political reputation has come from his leadership in the weeks following the attacks.

Like the other top Republican contenders, Arizona Sen. John McCain and former Massachusetts Gov. Mitt Romney, Giuliani has supported Bush's increase of U.S. troops in Iraq.

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