WASHINGTON, Sept 14 (Reuters) - Republican U.S. presidential front-runner Rudy Giuliani bought a full-page advertisement in The New York Times on Friday accusing the leading Democrat, Sen. Hillary Clinton, of "spewing political venom" for comments she made in the debate over the Iraq war.

The unusual and vituperative ad followed a week of partisan exchanges over President George W. Bush's policy of maintaining high levels of U.S. troops in Iraq, based on an assessment by the U.S. commander Gen. David Petraeus.

Clinton, who has called for early withdrawal of U.S. troops, said during a Senate hearing with Petraeus on Tuesday that his report outlining progress in the war required "a willing suspension of disbelief."

Giuliani, who built his reputation as a former mayor of New York where he was tough on crime and praised for his leadership after the Sept. 11 attacks, has backed Bush's policy of vigorously prosecuting the war.

In the ad he declared: "These times call for statesmanship, not politicians spewing political venom."

Giuliani also criticized Clinton for not denouncing a full page ad in the Times by the anti-war liberal group MoveOn.org on Monday that accused Petraeus of "cooking the books" on Iraq and called him "General Betray Us."

"Who should American listen to ... a decorated soldier's commitment to defending America, or Hillary Clinton's commitment to defending MoveOn.org?" said the Giuliani ad.

Skepticism over the progress of the war was expressed by many Democrats and some Republicans in Congress, although a week of appearances by Petraeus, Bush and other officials appears to have blunted demands for a radical policy change.

The MoveOn.org ad was widely criticized by Republicans, who have often accuse Bush's critics of accepting surrender and those who question assessments by U.S. military leaders of being unpatriotic.

The Giuliani ad, run 14 months before the 2008 election, was an unusual tactic at a time when candidates have been using mainly television and radio ads. It also targeted a Democratic candidate rather than Giuliani's Republican challengers for their party's presidential nomination.

In early opinion polls, both Giuliani and Clinton lead the races for their respective party's presidential nominations. The two almost clashed in 2000 for the New York Senate race, but he decided against a bid because of his fight against prostate cancer.

The Clinton campaign shrugged off the criticism, saying she would still work to end the war in Iraq and bring U.S. soldiers home as quickly as possible.

"Mayor Giuliani supports George Bush's Iraq policy and believes it is working," said Clinton spokesman Phil Singer. "Senator Clinton knows it isn't and will keep up her efforts to end the war."

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