(Adds Clinton comment about McCain)

By Andy Sullivan

NORFOLK, Va., Feb 8 (Reuters) - Sen. John McCain, his victory as Republican nominee for the U.S. presidency virtually assured, turned his sights on his Democratic challengers on Friday, saying they were weak on national security and their Iraq stance would hand al Qaeda a victory.

McCain's remarks, and the response from Democratic presidential contenders Barack Obama and Hillary Clinton, pushed Iraq war policy back to center stage in the presidential race after weeks of focus on the faltering U.S. economy.

Speaking to reporters after a security round-table meeting in Norfolk, Virginia, the home of a major U.S. naval base, McCain accused Obama and Clinton of wanting to set a date for withdrawing U.S. troops from Iraq.

"I believe that would have catastrophic consequences," said the Arizona senator, a former Navy pilot and prisoner of war in Vietnam. "I believe that al Qaeda would trumpet to the world that they had defeated the United States of America, and I believe that therefore they would try to follow us home."

He said the two Democrats, who have both pledged to quickly begin removing troops from Iraq if they are elected, had a "fundamental misunderstanding" of the issues at stake and would not admit the U.S. strategy to reduce violence had worked.

"They even refuse to acknowledge that. That I think is going to be a major issue in this campaign," said McCain, who criticized the military cutbacks under the past two administrations and said the armed forces needed to be expanded dramatically to ease the burden on those serving now.

McCain was virtually assured the Republican presidential nomination on Thursday after his chief rival withdrew, leaving former Arkansas Gov. Mike Huckabee, who is trailing far behind, as his main challenger.

DEMOCRATS STRIKE BACK

The Obama and Clinton camps, campaigning in Washington state ahead of Saturday's new round of state-by-state contests to choose Republican and Democratic candidates for the November election, fired back at McCain while arguing they were best suited to run against him.

Obama, an Illinois senator who would be the first black U.S. president, told a Seattle news conference the maverick Arizona senator, who has had trouble rallying the support of the Republican Party's conservative base, had been wrong in his support of the Iraq war.

"On the most important foreign policy decision in perhaps a generation, I strongly believe John McCain got it wrong," Obama said.

"I think it has been an enormous strategic blunder on the part of the United States. It has made us less safe. It has cost us dearly in blood and treasure," he added.

Clinton, a New York senator and former first lady who would be the first woman U.S. president, said in Spokane, Washington, she was the strongest candidate to take on McCain in November.

"We have to have a candidate who can go toe-to-toe with Senator McCain on national security ... on the economy and health care and everything else. I believe that I am the best prepared," Clinton said.

Clinton, who has been lagging Obama in fundraising and recently loaned her campaign $5 million, asked people at an earlier event in Tacoma, Washington, to contribute and said she had been overwhelmed by donations from new contributors, including 75,000 people who visited her Web site in recent days.

Her campaign deflected Obama's suggestions after she made the loan that she should release her tax information. Spokesman Howard Wolfson said Clinton would disclose her taxes if she won the nomination and needled Obama over his links to an indicted Chicago businessman.

Obama and Clinton face off in weekend races in Louisiana, Nebraska, Washington and Maine to choose Democratic presidential nominees. (Additional reporting by Caren Bohan and Jeff Mason, writing by David Alexander; editing by David Wiessler and Eric Beech) (To read more about the U.S. political campaign, visit Reuters "Tales from the Trail: 2008" online at http://blogs.reuters.com/trail08/

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