ORLANDO, Fla. — Sen. John McCain, R-Ariz., challenged Democratic Illinois Sen. Barack Obama's readiness to be president and understanding of military affairs yesterday, telling a national veterans' convention that the gains of America's troops in Iraq are at risk if the Democratic candidate becomes president.

In a speech to the annual gathering of the Veterans of Foreign Wars, McCain portrayed Obama as a naif motivated by ambition.

"The hard-won gains of our troops hang in the balance" as the country weighs its choice for president, McCain said, criticizing Obama's plan for withdrawing troops from Iraq within 16 months. "The lasting advantage of a peaceful and democratic ally in the heart of the Middle East could still be squandered by hasty withdrawal and arbitrary timelines."

Faulting Obama for "shifting" statements on the Iraq troop surge, McCain said that behind his rival's positions "lies the ambition to be president."

"What's less apparent is the judgment to be commander in chief," McCain continued. "And in matters of national security, good judgment will be at a premium in the term of the next president — as we were all reminded 10 days ago by events in the nation of Georgia."

Obama, who spent the day emphasizing the economy in the battleground state of New Mexico, returned fire during a town hall-style meeting in Albuquerque. He accused McCain of questioning his patriotism with the suggestion that Obama would rather lose a war than an election.

"They've been doing this in every election," Obama said. "They did it to (Massachusetts Sen.) John Kerry, they did it to (former Vice President Al) Gore, they tried to do it to (former President) Clinton, they did it to (former Massachusetts Gov. Michael) Dukakis," Obama said. "That's what they do, that's their politics. They don't know how to govern, they know how to run a negative campaign."

The Illinois senator said negative campaign tactics would not derail his candidacy.

"It's not going to work this time," he said. "In order for us to win, we not only have to beat the attack machine, we've got to win back the trust of the American people."

The day's back-and-forth illustrated the different focus of the two candidates as they head into their respective nominating conventions, which begin next week when the Democrats gather in Denver.

While Obama has pressed his argument that working-class and middle-class Americans have suffered under President Bush's economic policies, McCain has seized on the clash between Russia and Georgia to underscore what he claims is the foreign policy experience gap between himself and his younger rival.