WASHINGTON — President Bush, in a preview of what a McCain presidency might look like, is saying he will veto any Iraq war funding bill that imposes a timeline for withdrawal or includes unnecessary spending.

In a White House speech yesterday wrapping up a week in which General David Petraeus and Ambassador Ryan Crocker reported to Congress on security and political progress in Iraq, Mr. Bush set out his conditions for funding the war in his last year in office.

“Members of Congress must pass a bill that provides our troops the resources they need — and does not tie the hands of our commanders or impose artificial timelines for withdrawal,” he said. “This bill must also be fiscally responsible. It must not exceed the reasonable $108 billion request I sent to Congress months ago. If the bill meets all these requirements, it will be a strong show of support for our troops. If it doesn’t, I’ll veto it.”

The tough warning was preceded by a concession of sorts: Mr. Bush announced that an anticipated withdrawal of five brigades from Iraq would mean that tours of deployments for Army soldiers would be reduced to 12 months from 15.

But Mr. Bush said he and General Petraeus would consider further troop withdrawals based on conditions on the ground in Iraq and not promise a larger reduction in forces.

That position ends a months-long debate in Washington. Secretary of Defense Gates and the recently retired head of U.S. Central Command, Admiral William Fallon, both favored deeper cuts to troop levels in Iraq, as do leading Democrats in Congress and the two senators vying for the Democratic presidential nomination, Hillary Rodham Clinton and Barack Obama.

Mr. Bush said yesterday that it was wrong to call his approach to troop reductions a “pause.” “Some have suggested that this period of evaluation will be a ‘pause,'” he said. “That’s misleading, because none of our operations in Iraq will be on hold. Instead, we will use the months ahead to take advantage of opportunities created by the surge, and continue operations across the board.”

In 2007, Senator Webb, a Democrat of Virginia and secretary of the Navy under President Reagan, co-authored legislation to mandate a shorter troop deployment schedule. At the time, the White House opposed the bill, saying shorter tours would interfere with the schedule for sending reinforcements for its surge strategy. Democrats yesterday seized on the president’s announcement on rotation tours.

“Democrats welcome President Bush’s announcement today to limit future army deployments to 12 months, but believe more must be done to limit the strain on our forces currently deployed and relieve the strain on our marines,” the Senate majority leader, Harry Reid of Nevada, said. “Now that President Bush has finally acknowledged the strain on our forces, will Bush Republicans join Democrats in codifying the announcement into law and ensuring that our troops have the dwell time they need to recover from combat and train for future operations?”

The chairman of the House Armed Services Committee, Rep. Ike Skelton, a Democrat of Missouri, also praised Mr. Bush’s announcement on the deployment schedule. But he said: “Our Army has become overstretched and becomes more overextended with each passing day. We cannot afford to take on more strategic risk to buy more time for a government uninterested in reconciliation.”

In his speech on the war, Mr. Bush also singled out Iran. He gave the regime in Tehran a choice: “It can live in peace with its neighbor, enjoy strong economic and cultural and religious ties. Or it can continue to arm and train and fund illegal militant groups, which are terrorizing the Iraqi people and turning them against Iran. If Iran makes the right choice, America will encourage a peaceful relationship between Iran and Iraq. Iran makes the wrong choice, America will act to protect our interests, and our troops, and our Iraqi partners,” he said.

The Bush administration’s concern about Iran was echoed yesterday by Vice President Cheney, who called President Ahmadinejad a “very dangerous man.” Speaking on the Sean Hannity radio show, Mr. Cheney pointed out that the Iranian president has stated his intention to destroy Israel and believes in the return of the 12th imam.

The vice president then quoted a theory by the Middle East historian Bernard Lewis that Mr. Ahmadinejad seeks to herald the apocalypse. “Bernard Lewis once said, ‘Mutual assured destruction in the Soviet-U.S. relationship in the Cold War meant deterrence, but mutual assured destruction with Ahmadinejad is an incentive.’ You have to be concerned about that,” Mr. Cheney said.