(Updates with Obama comments)

By Jeff Mason

WASHINGTON, April 8 (Reuters) - U.S. presidential candidates John McCain, Hillary Clinton and Barack Obama pushed competing positions on the Iraq war at congressional hearings on Tuesday in political theater that may define the race for the White House.

Speaking at televised sessions with Gen. David Petraeus, the top U.S. commander in Iraq, Republican McCain, a senator from Arizona, said combat forces should stay in the country.

Democrats Clinton and Obama, senators from New York and Illinois, respectively, said American troops should withdraw.

All three senators repeated well-known positions, but the high-profile setting and one-day departure from the campaign trail made for a closely watched political show.

Obama, who has emphasized his long held opposition to the war and says he would withdraw troops immediately after entering office, said a timetable for withdrawal was necessary to pressure Iraqi leaders to bring about peace.

"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," Obama said.

Clinton, who has been criticized by Obama for her vote in the Senate to authorize the use of force in Iraq, has promised to begin drawing down troops within 60 days of becoming president.

"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," she told a hearing of the Senate Armed Services Committee.

McCain, who has locked up the Republican nomination for the November election to succeed President George W. Bush, warned that a withdrawal could require U.S. troops to return in a broader war.

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

BACK ON CENTER STAGE

The war in Iraq has largely taken a back seat on the campaign trail to worries about the softening U.S. economy, but the Petraeus' testimony put it back on center stage this week where it is likely to be during the upcoming general election.

While the candidates stuck mostly to substantive questions, their supporters contributed to the political show.

McCain backer Lindsey Graham, a Republican senator from South Carolina, pushed Petraeus to spell out the consequences of an withdrawal advocated by many Democrats.

"I have advocated conditions-based reductions, not a timetable," Petraeus responded. "War is not a linear phenomenon; it's a calculus, not arithmetic."

Indiana Sen. Evan Bayh, a Democrat who has been mentioned as a possible vice presidential candidate for Clinton, pushed back against suggestions that calling for a timetable for withdrawal was defeatist.

"As I acknowledge your honor and patriotism, I hope you would acknowledge the honor and patriotism of those who look at this very complex set of facts and simply have a very different point of view," Bayh told the general.

Petraeus responded: "Senator, we fight for the right of people to have other opinions." (Additional reporting by Andy Sullivan and David Storey; editing by Chris Wilson)

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