April 8 (Reuters) - U.S. presidential candidates John McCain, Hillary Clinton and Barack Obama pushed competing positions on Iraq during hearings on Tuesday with Gen. David Petraeus, the top U.S. commander in Iraq.

Here is a sampling of quotes from the candidates:

ARIZONA SEN. JOHN MCCAIN, REPUBLICAN

"We're no longer staring into the abyss of defeat and we can now look ahead to the genuine prospect of success."

If U.S. troops withdraw before stability is established, "we will exchange for this victory a defeat that is terrible and long-lasting. Al Qaeda in Iraq would proclaim victory, increase its efforts to provoke sectarian tensions, pushing for a full-scale civil war that could descend into genocide and destabilize the Middle East."

NEW YORK SEN. HILLARY CLINTON, DEMOCRAT

"I think it's time to begin an orderly process of withdrawing our troops, start rebuilding our military and focusing on the challenges posed by Afghanistan, the global terrorist groups and other problems that confront Americans."

"The (Bush) administration and supporters of the administration policy often talk about the cost of leaving Iraq, yet ignore the greater costs of continuing the same failed policy."

ILLINOIS SEN. BARACK OBAMA, DEMOCRAT

"Increased pressure in a measured way, in my mind ... includes a timetable for withdrawal. Nobody's asking for a precipitous withdrawal, but I do think that it has to be a measured but increased pressure."

"I'm not suggesting that we yank all our troops out all the way. I'm trying to get to an endpoint." (Reporting by Andy Sullivan and Jeff Mason, editing by Todd Eastham)

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