Without additional troops to ensure victory in Iraq, the U.S. could find itself more vulnerable to terrorist attacks at home, Sen. John McCain said Sunday.

Taking the opposite tack, newly empowered Democrats pressed their case for a phased withdrawal of American forces. They hoped a blue-ribbon advisory panel would propose a way ahead for Iraq, while making clear the U.S. military mission shouldn't last indefinitely.

McCain, a front-running GOP presidential hopeful for 2008, said the U.S. must send an overwhelming number of troops to stabilize Iraq or face more attacks — in the region and possibly on American soil.

"I believe the consequences of failure are catastrophic," said McCain, R-Ariz. "It will spread to the region. You will see Iran more emboldened. Eventually, you could see Iran pose a greater threat to the state of Israel."

With about 141,000 U.S. troops in Iraq more than 3 1/2 years into the war, the American military has strained to provide enough forces while allowing for adequate rest and retraining between deployments.

But McCain, who spent 5 1/2 years as a prisoner of war after his Navy plane was shot down in 1967, recalled the Vietnam War's lessons. "We left Vietnam. It was over. We just had to heal the wounds of war," he said. "We leave this place, chaos in the region, and they'll follow us home. So there's a great deal more at stake here in this conflict, in my view, a lot more."

McCain said he based his judgment partly on the writings of Abu Musab al-Zarqawi, the Al Qaeda leader in Iraq who was killed in a U.S. air raid, and of Usama bin Laden.

"The consequences of failure are so severe that I will exhaust every possibility to try to fix this situation. Because it's not the end when American troops leave. The battleground shifts, and we'll be fighting them again," McCain said. "You read Zarqawi, and you read bin Laden. ... It's not just Iraq that they're interested in. It's the region, and then us."

Sen. Joseph Biden, the incoming chairman of the Senate Foreign Relations Committee, said he hoped a special commission considering options for the way ahead in Iraq would assert that U.S. troop commitments are not open-ended; propose a clear political road map for Iraq; and recommend engaging Iraq's neighbors in a political solution.

"We are past the point of adding more troops. We are past the point of vague policy prescriptions. It is not an answer just to stay. Nor is it an answer — though it may become a necessity — just to go with no concern for what follows," Biden, D-Del., wrote in Sunday's Washington Post.

"The fundamental question we must answer is whether, as we begin to leave Iraq, there are still concrete steps we can take to avoid leaving chaos behind," said Biden, who plans to run for president in 2008.

Democrats poised to take control of the House and Senate are pressing for a substantial reduction of U.S. troops in Iraq and a timetable for their withdrawal, as a way of forcing the Iraq government to rely more on itself.

"We must tell the Iraqis that we would begin, starting in four to six months, a phased reduction of our troops," said Sen. Carl Levin, D-Mich., the incoming chairman of the Senate Armed Services Committee. "Because if you don't do that, they're going to continue to have the false assumption that we are there in some kind of an open-ended way. And it is that assumption on their part that takes them off the hook."

Incoming House Majority Leader Steny Hoyer, D-Md., agreed. "As a practical matter, there are no troops to increase with," he said. "Our objective was to remove Saddam Hussein and create an environment in which a democracy could be established. That has been done."

But Hoyer said Democrats would continue funding the existing troop levels, for the time being.

"That's not an option, of not supporting our troops in the field and making sure they're as safe as we can make them," he said. "Very frankly, their lack of numbers exposes them on a daily basis to danger and death, unfortunately. But clearly, we're going to have discussions going forward as to how we change this policy and change it in the short term, not the long term."

Gen. John Abizaid, the top U.S. commander for the Middle East, told the Senate Armed Services Committee last week he believes troop levels should remain steady for now. He said it was possible to add 20,000 troops for a short time, but it would be unrealistic to raise troop levels as proposed by McCain and Sen. Lindsey Graham (news, bio, voting record), R-S.C.

Abizaid said the American military in Iraq is stretched too thin already, and sending over a bigger, more permanent presence would undercut efforts to force Iraqis to take on more responsibility.

Sen. John Kerry, D-Mass., said he thinks U.S. generals have been put "in a a very, very difficult position."

"What I don't have confidence in is the policy. And General Abizaid is giving us a diagnosis that is based on the current policy. But that policy has to change, and it can change," said Kerry, who is considering a White House bid in 2008.

Kerry spoke on "FOX News Sunday," while McCain and Hoyer appeared on ABC's "This Week." Levin spoke on CNN's "Late Edition." Graham appeared on "Face the Nation" on CBS.