SPRINGFIELD, Ill. — Which Barack Obama will show up for the first presidential debate?

It could be the tone-deaf debater who condescendingly told Hillary Rodham Clinton during a Democratic debate that she was "likable enough."

Or perhaps the confident candidate who absorbed a jab from Clinton about using her husband's former advisers and responded with a devastating one-liner of his own: "Hillary, I'm looking forward to you advising me as well."

For a man known as a powerful speaker, Obama has rarely wowed people in political debates. He can come across as lifeless, aloof and windy.

But Obama didn't make any serious mistakes in the many debates during the Democratic primary, or when he was running for the U.S. Senate in Illinois. He sometimes showed flashes of wit and charm. And, with a couple of exceptions, he got better with time.

"A year ago, he was not nearly as polished," said Timothy O'Donnell, a professor at the University of Mary Washington and chairman of the collegiate National Debate Tournament. "He equivocates less. He's quicker with examples."

STAYING ON OFFENSE

O'Donnell says staying on offense will be key if Obama wants to shape the discussion and reach undecided voters.

The Illinois senator failed to do that in what is often mentioned as his worst performance in a major debate, an April 16 confrontation with Clinton in Philadelphia.

With Obama on the verge of wrapping up the nomination, the moderators focused on his potential weaknesses, asking questions about Obama's former minister, his policy on flag lapel pins and his comments about rural people clinging to guns and religion.

Obama seemed deflated by the questioning and failed to steer the debate toward his theme of change. His performance did little to reassure nervous supporters.

In another debate, Obama was asked how he'd respond militarily if terrorists attacked two American cities simultaneously. Rather than display any passion, Obama discussed emergency response procedures and intelligence-gathering.

A flat performance is one thing; the big worry is a single thoughtless remark that sticks in voters' minds. Obama still catches flak for a few remarks from the primary debates.

In last year's YouTube debate, Obama said he would be willing to meet the leaders of nations such as Cuba, Iran and North Korea without preconditions.

His Democratic rivals pounced, calling the stance naive and dangerous. Obama adviser David Axelrod insisted Obama was talking only about midlevel diplomatic discussions, not summit meetings involving the president.

But Obama decided to stand by his statement, using it to draw a distinction between himself and candidates with more conventional approaches to diplomacy. Opponents have used it ever since to argue that Obama would be soft in dealing with dangerous nations.

HUMOR CAN BE FLAT

Another memorable moment came just before the New Hampshire primary, when Clinton was being quizzed about whether voters liked her less than Obama. She deflected the question by joking that her feelings were hurt and admitting that Obama was very likable.

Barely looking up from his notes, Obama broke in to say, "You're likable enough, Hillary. No doubt about it." It may have been a dry attempt at humor, but it fell flat.

Obama has used humor effectively in other debates.

When asked how he felt about Bill Clinton being described as the first African-American president, Obama gave a serious answer about civil rights and overcoming racism. Then he added, "I have to say that, you know, I would have to investigate more of Bill's dancing abilities and some of this other stuff before I accurately judged whether he was, in fact, a brother."

The audience roared.

O'Donnell said Obama has displayed "small glimpses" of the ability to steer a damaging debate toward a more favorable tone and message. In one debate, he took a question about declaring English the country's official language and turned it into a discussion about dividing Americans rather than bringing them together.

Obama also can cut his opponents with a single icy remark.

In 2004, Republican Senate candidate Alan Keyes suggested Obama couldn't follow his rival's logic on a convoluted point about gay rights.

"Your logic wasn't that complicated," Obama shot back. "It was just wrong."