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John Kerry, a decorated Navy veteran criticized by Republicans for his anti-war activities during the Vietnam era, lashed out at President Bush on Monday for failing to prove whether he fulfilled his commitment to the National Guard (search) during the same period.

Conservative critics have questioned whether Kerry deserved his three Purple Hearts (search) for battle wounds, an issue the Democratic presidential candidate sought to put to rest last week by releasing his military records. On Sunday, a top Bush adviser criticized Kerry for leading anti-war protests after he returned from the battlefield.

"If George Bush wants to ask me questions about that through his surrogates, he owes America an explanation about whether or not he showed up for duty in the National Guard. Prove it. That's what we ought to have," Kerry told NBC News in an interview. "I'm not going to stand around and let them play games."

Kerry's direct criticism of Bush's Guard record reflected an aggressive strategy to challenge the president. It came as Vice President Dick Cheney (search) used a speech in Missouri to question Kerry's fitness to be commander in chief, and the Bush campaign launched a $10 million television ad campaign criticizing Kerry's Senate votes on weapons systems.

During the primaries, Kerry often deflected questions about Bush's military service although when asked in February whether Bush had fulfilled his Vietnam-era commitment, the Democrat said, "Just because you get an honorable discharge does not in fact answer that question."

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In 1992, as Democratic candidate Bill Clinton (search) faced criticism for avoiding service in Vietnam, Kerry said, "We do not need to divide America over who served and how. I have personally always believed that many served in many different ways."

Democrats have questioned whether Bush fulfilled his obligations to the National Guard in spite of White House claims that he completed his duty satisfactorily. Bush joined the Texas Air National Guard in 1968 and transferred to the Alabama Guard in 1972 while working on a political campaign. How often Bush reported for duty in Alabama is unclear.

Bush supporters have tried to turn Kerry's service in Vietnam — a centerpiece of his Democratic campaign — against him even as they say they honor his service to his country. Kerry released his medical records when questioned about the extent of his war wounds, including a report showing he still carries shrapnel in one leg.

That criticism silenced for the moment, Bush adviser Karen Hughes turned to what Kerry did after returning from Vietnam. Hughes said Sunday she was offended by Kerry's anti-war activities in 1971 and accused him of not throwing back his medals when he and other veterans protested in Washington.

"He only pretended to throw his," Hughes said in a CNN interview. "Now, I can understand if, out of conscience, you take a principled stand, and you would decide that you were so opposed to this that you would actually throw your medals. But to pretend to do so — I think that's very revealing."

Kerry has never said he pretended to throw away his medals. For years, he has said that he threw his ribbons over a fence at the Capitol, not his three Purple Hearts, Bronze Star and Silver Star. He also has said that after the protest he threw the medals of two other veterans.

Nearly 800 veterans "gave back" their medals, ribbons, dog tags and other military items during a protest in April 1971. However, a tape of a television interview Kerry gave shortly after the protest suggested he had claimed that he also threw his medals.

In the exchange, aired Monday by ABC and published in The New York Times, an interviewer asks Kerry, "How many did you give back, John?" Kerry responds, "I gave back, I can't remember, six, seven, eight, nine." The host then notes that Kerry had won the Purple Hearts, and Bronze and Silver stars. Kerry says, "Well, and above that, I gave back my others."

Kerry told ABC on Monday that the terms ribbons and medals were interchangeable. He accused Republicans of trying to discredit his presidential campaign with a "phony controversy."

"The U.S. Navy pamphlet calls them medals," he said. "We referred to them as the symbols, they were representing medals, ribbons. Countless veterans threw the ribbons back."

Kerry was asked to reconcile two explanations for why he didn't throw his own medals: He told The Washington Post in 1985 it was because he didn't want to personally, and told the Boston Globe in 1996 that he didn't have time to go home and get them.

"I've expressed that there was great, sort of, sense of wrenching about the whole thing," Kerry told ABC's "Good Morning America" in an interview Monday. He noted that the anti-war veterans were conflicted over whether to throw them, and although they voted to do so, "I threw my ribbons. I didn't have my medals. It's very simple."

The controversy over the medals overshadowed the start of Kerry's three-day bus tour of four manufacturing states that are expected to be pivotal in this year's election — West Virginia, Pennsylvania, Ohio and Michigan. Kerry began the tour in Wheeling, W.Va., with a speech accusing Bush of failing to enforce trade rules that protect U.S. workers.