"If we can't trust Al Gore on debates, why should we trust him on anything?" asks a narrator in the ad.

But it was unclear how many viewers would actually see it. Campaign officials were debating whether to make a modest buy in a handful of markets or to rotate it fully with spots now airing on education and Social Security in 21 battleground states, according to a GOP official.

The Texas governor also sought to raise fresh doubts about Gore on military readiness.

Speaking at the American Legion's annual convention in Milwaukee, he repeated his earlier contention that the U.S. military was "in decline."

Gore has criticized Bush in turn, saying that the U.S. military is by far the strongest in the world but that political talk about weakness can only help potential foes.

"These are not criticisms of the military," Bush said Wednesday. "They are criticisms of the current commander in chief and the vice president for not providing the necessary leadership."

Bush, a pilot in the Texas Air National Guard during Vietnam, wore a blue service cap from an American Legion Post in Houston.

"George Bush, Post 77, reporting for duty," he told members of the 2.8-million service organization.

Gore, who served as an Army journalist in Vietnam, did not accept the American Legion's invitation, citing a scheduling conflict.

On the debates, Bush said earlier in the day in Milwaukee, "I'm still showing up" at the NBC studio in Washington on Tuesday in expectation of a joint appearance with Gore. "I hope the vice president shows up."

Bush has agreed to do only one of three 90-minute prime-time debates proposed by the bipartisan commission that has run presidential debates since 1988.

Instead, citing offers by Gore to debate him in dozens of other locations, Bush accepted an invitation for a special evening edition of NBC's "Meet the Press" next Tuesday and for a later session on CNN's "Larry King Live."

The Gore campaign has refused to do the talk show sessions unless Bush first agrees to the full set of commission debates.

Gore suggests Bush is dodging debates that would be seen by large audiences. The Republicans suggest Gore's stance shows he is untrustworthy.

"When it was politically convenient, Gore said he'd debate anytime, anyplace, anywhere," an announcer says in the new 30-second spot. "Now that Governor Bush has accepted, Gore says 'unacceptable."'

Bush spokesman Ari Fleischer said the ad was electronically shipped to stations Wednesday, although a check of stations in several states found that none of them had received it yet.

The Bush campaign is spending about $5.5 million per week on ads.

"Rather than spending millions on attack ads, George W. Bush should follow his father's lead and agree to the bipartisan proposal for debates," said Gore spokesman Doug Hattaway.

Fleischer insisted the new ad did not break Bush's promise of a positive campaign. "It's accurate. The governor has always been explicit that credibility was an issue in this race," he said.

With officials unsure how many times they would actually air the commercial, the ad seemed designed as much to attract news coverage.

Democrats cried foul: "If you are going to go throwing charges of credibility around, you better make sure you're credible," said Jenny Backus, spokeswoman for the Democratic National Committee.

During a round of satellite interviews, Gore stood by his insistence on commission debates, but he added, "If the Bush campaign has a problem with a specific format you know, we're open to all that."

Green Party presidential candidate Ralph Nader, meanwhile, complained that both Gore and Bush would "effectively deny major third-party candidates a voice in the debates." He has sent a letter to the networks asking to be included.

Trying to stem Gore's recent advance in the polls, Bush campaigned in four states on Wednesday: Pennsylvania, Wisconsin, Indiana and Michigan.

At an airport rally in Indianapolis, Bush said Gore would not serve average Americans. "Mr. Vice President, you are the powerful. You've been in power for eight years. You've had your chance."