“You like to pretend like the war started in 2007  you talk about the surge. The war started in 2003,” Mr. Obama said. “At the time, when the war started, you said it was going to be quick and easy. You said we knew where the weapons of mass destruction were. You were wrong. You said that we were going to be greeted as liberators. You were wrong.”

There were no obvious game-changing moments  big mistakes, or the kind of sound bites that dominate the news for days  in the course of the 90-minute debate, held at the University of Mississippi in Oxford. Still, the debate served as a reminder of just how different these two men would be as president as they appeared for their first extended session together before a huge audience, including many Americans who are just beginning to focus on this long-lasting race.

The differences were in no small part stylistic and visible with a glance to the stage: a 47-year-old black man who has been in the Senate for nearly four years standing at one lectern, facing a 72-year-old white-haired fixture of the Senate standing across from him. In many ways, Mr. Obama was a very different candidate than he was during the primary battles. He answered questions directly and affirmatively, typically looking right into the camera as he spoke.

Throughout the debate, Mr. Obama called Mr. McCain by his first name; Mr. McCain did not. The direct engagement was encouraged by the moderator, Jim Lehrer of PBS, who declared, “I’m just determined to get you all to talk to each other,” though it was an invitation that the two men repeatedly ignored.

Mr. McCain was feisty and aggressive but, particularly during the start of the debate, his language and demeanor offered a reminder of just how much he was a creature of the Senate, as he used phrases that were no doubt understandable in Washington but might have been lost to the audience at home. He spent much of the first 20 minutes of the debate criticizing Mr. Obama for supporting earmarks, special projects sought by members for their district.

“The United States Senate will take up a continuing resolution tomorrow or the next day  sometime next week  with 2,000  2,000  look at them, my friends,” he said. “Look at ’em. You’ll be appalled. And Senator Obama is a recent convert, after requesting $932 million worth of pork-barrel spending projects.”

On Iraq, both candidates used the stories of fallen soldiers to support their own positions on the war. Mr. McCain told the audience about a New Hampshire woman who presented him with a bracelet of her 22-year-old son who was killed in combat. She asked him to keep alive the mission so his death was not in vain.