Contrasts on the campaign trail run the gamut, from the way they stage events and draw crowds to how they court voters and handle reporters.
The two men even carry themselves differently.
McCain, 5-foot-9 and spry at age 71, has a ramrod posture, quick movements and clenched fists; he darts from here to there as if spurred forward by jolts of energy. The combination can make him come across as stiff, despite his remarkably informal and off-the-cuff approach.
Obama, age 46 and lanky at 6-foot-2, seems to glide, his head held high, his gait long and his arms swinging freely at his sides if not buried in pockets. His laid-back mannerisms can seem overly casual, and they belie his highly scripted and picture-perfect appearances.
The divergent personal styles and campaign approaches, particularly during the primary season, offer a glimpse of a possible matchup this fall should Obama win the Democratic nomination over Hillary Rodham Clinton and go head-to-head with McCain, the GOP nominee-in-waiting. At this point, Obama leads Clinton in pledged delegates; it is mathematically difficult for her to win.
For any candidate, mannerisms on the trail can provide insights into how one could behave in office.
Bill Clinton adored glad-handing with voters so much that the Democrat frequently ran late as a candidate; he didn't change as president. George W. Bush was known to josh and needle campaign trail reporters; the Republican does the same with the White House media.
Now, in 2008.
JOHN MCCAIN: The Arizona Republican, who would be the nation's oldest man to be elected president, was arguably the most open White House candidate in modern history during the GOP primary fight.
That's changing some now that he's the party's likely nominee; he's giving more choreographed speeches behind podiums and with TelePrompTers.
Still, most of the time, McCain is incredibly unscripted. And, that can be dangerous: his mouth - and his temperament - can get him in trouble and give Democrats fodder to criticize. One instance: he jokingly sang "bomb Iran" to the tune of the Beach Boys "Barbara Ann."
Nevertheless, McCain still favors the low-key give-and-take of a town-hall style setting. He prefers to be on the same level as his audience and grimaces under the glare of lights. His only prop is typically a microphone, cordless so he can wander - or pace - in front of his audience and venture into the aisles to hand it to folks.
Witty and quick, he often opens events with a couple of jokes, not all funny. He gives a synopsis of where he stands on issues, a policy-laden monologue setting the stage for a policy-laden discussion thereafter. He tells his crowds to ask anything, calls on people one by one and encourages them to challenge his point of view.
"Follow-ups are allowed," he says, and prods: "Go ahead."
When people stand up and praise his heroism in Vietnam, the former prisoner of war thanks them - and then quickly asks if they have a question. When protesters shout out, McCain puts them on the spot and creates a mini-debate atmosphere.
"Just a couple more. Just a couple more," McCain always pleads when an aide tries to end the questioning. He usually gets his way. And then, when the mike is off, he wades into his audience, shaking hands, and chatting people up. They don't fawn over him, but simply offer words of respect and gratitude for his years of service.
Even now, as he segues into the general election, his venues are intimate - community centers, Veterans of Foreign Wars halls. That's partly out of preference, partly out of necessity; his crowds are far smaller and less energetic than those of his Democratic rivals.
After events, McCain almost always takes media questions. Then, it's back to his bus, where McCain parks himself in a leather captain's chair with a scrum of reporters squeezed in around him, balancing their tape recorders on his knees. He'll spend hours talking with them about anything, sometimes leaving reporters with little else to ask and the candidate pushing: "What else? What else?"
Recently, McCain has spent more time on his plane, and the tighter, louder quarters have limited his access. He says he's not happy about it: "It's a lot more fun on the bus."
BARACK OBAMA: The Illinois Democrat, competing to be the first black president, has generated an enormous following unseen in decades for a presidential candidate. His appearances tend to be carefully planned. The approach can invite criticism that he's all style and little substance, as well as raise questions about whether he's ready to be in the hot seat.
Giant outdoor rallies, rain or shine, and sprawling events at indoor arenas to accommodate thousands upon thousands have tended to be the norm, with an occasional question-and-answer event for voters.
That was the case in the days leading up to the Pennsylvania primary. Some 35,000 people showed up in Philadelphia one night. The next day found Obama on a whistle-stop tour, where he spoke to tens of thousands more people at five different rallies. A couple of high school gymnasium events followed, and, there, Obama took some questions from the audience.
At each stop, Obama sauntered up on the stage as rock music blared and supporters cheered. A gifted orator, he was usually elevated in the middle of the crowd and a podium was often in place. Even so, he would meander across the stage as he gave his talk about a need to change the ways of Washington and do away with old-style politics. With a natural charisma, he was clearly comfortable being the center of attention and talking to voters. The crowds were always rambunctious, engaging with the candidate as he stood above them.
"This is now our moment. This is now our turn," Obama declared repeatedly in an inspirational tone.
As he spoke, people on all sides of him would yell "We love you!" and Obama would respond with a wave and a smile, or a fingerpoint, "I love you guys, too!" He sprinkled his pitch with, "So, you know what I'm talkin' about" and "that's right" when the crowd cheered in agreement.
Afterward, with his full Secret Service detail in tow and limiting his movements, he worked the metal barriers that kept the crowds at bay, shaking hands, giving out hugs, posing for pictures and signing autographs. Many fans stretch their arms out to try to touch him; some end up in tears.
He was hardly as open with the media; at one point, it had been more than a week since Obama answered the questions of reporters who traveled with him. He did do some interviews with local media, though all but ignored the national media, save for buying reporters a couple of pies at a family style restaurant after greeting voters there.
At one diner stop, Obama bristled when a reporter tried to ask him a question: "Why can't I just eat my waffle?"
In a sign of what may be coming, Obama started taking a different approach this week: he's opted for smaller settings with voters and he's been more available to the media.