Barack Obama's coolness cuts both ways.

Before Mr. Obama's March 18 race-relations speech, top adviser David Axelrod went to sleep unsure what the candidate would say the next morning. When a restless Mr. Axelrod awoke at 2 a.m., the entire speech, written by Mr. Obama, was waiting on his Blackberry.

"I thought, how could a guy operating under this kind of pressure, three hours of sleep a night, produce that kind of speech?" Mr. Axelrod said.

That's one side of cool. There's another, one that can verge on cold.

Last August, the Democratic presidential candidate got a surprise endorsement from Zbigniew Brzezinski, the national security adviser under President Jimmy Carter, boosting Mr. Obama's foreign-policy credentials. The next month, the campaign enlisted Mr. Brzezinski to introduce him at an event in Iowa.

Then, before the March 4 Ohio primary, Mr. Obama spoke privately with a group of Jewish leaders in Cleveland and dismissed any formal association with Mr. Brzezinski, who's seen by some as too critical of Israel. "He's not one of my advisers," Mr. Obama said. "I've had lunch with him once."

For the Illinois senator, cool can be complicated. It helps him maintain composure as he bursts through the barriers of race and politics; it also creates about him an aura of detachment.

It fuels his boldness as well as his caution, the inspiration and the calculation, the intelligence and the ambition that this week made him the first black presidential nominee of a major U.S. party.

While Mr. Obama, 47, has profited from no small measure of luck, it is his willingness to take risks -- seizing opportunities for advancement as soon as they have presented themselves -- that made his triumph possible. The man who titled his best-selling memoir "The Audacity of Hope" has benefited far more from audacity of his own.

The soaring quality of his message of "hope" and "change" propelled him to the nomination, its vagueness both a strength and weakness. The challenge of the Democratic convention and the campaign might well be how he fills in the canvass.

Mr. Obama has shown an "enormous ability to arouse the intense admiration and affection of his base," said Sean Wilentz, a history professor at Princeton University. "Exactly what he means by change, hope and transformation -- all the sort of big-payoff words that appear in his speeches -- he has yet to clearly define."

One reason may be his limited time in the public arena.

There's no modern American precedent for the rapid trajectory of his career, which has made him that rare politician who's also a global celebrity.

Mr. Obama is the first nominee of a major party in more than a half-century whose life wasn't shaped by World War II, Vietnam or the struggles for racial and gender equality of the 1960s. He is the first to have such an exotic biography, the son of a white woman from Kansas and a black man from Kenya who grew up in Hawaii and Indonesia and has relatives on four continents.

His barrier-breaking candidacy also came at a price: It ended the hopes of those who wanted to see Sen. Hillary Rodham Clinton of New York become the first woman to lead a major-party ticket. Though polls suggest Mr. Obama has captured some of her supporters, it's clear others remained unconvinced.

On the other hand, his candidacy has bridged some generational divisions. Robert Rubin, Treasury secretary under President Bill Clinton, initially supported Mrs. Clinton, while his son, Jamie, raised money for Mr. Obama. The father is now backing Mr. Obama too.

He also bridged a generational divide with his choice of Delaware Sen. Joseph R. Biden Jr., 65, as his vice-presidential running mate. The campaign hopes Mr. Biden will help inoculate Mr. Obama from criticism of his lack of foreign-policy experience, while helping him reach out to working-class voters who favored Mrs. Clinton in the primary.

Mr. Obama needs no such help among younger voters: His pull among them has earned him credentials as a digital-age candidate, harnessing the power of the Internet to greater effect than any rival to build a network of 2 million donors and raise a record $390 million as of July 31.

What kind of president he would be and how he'd make decisions are among the questions Mr. Obama will have to answer between now and November. Already, his policy shifts on issues from federal wiretapping legislation to Social Security taxes to election financing have led even some Democrats to question the firmness of his convictions.

So far, Mr. Obama has picked his battles carefully, choosing most often to focus on winning converts. "One of the dangers here is that in the effort to constantly adjust his position, people then begin to wonder where his bottom line is," said Leon Panetta, President Clinton's former chief of staff. "He's got to show that he's got a bottom line and that he's willing to fight for it."

His visits last month to Afghanistan, Iraq, the Middle East and Europe, testing the world's embrace of him as a potential president, drew crowds that peaked at 200,000 in Berlin. The positive reviews weren't limited to non-Americans.

"I've been in meetings with at least three different presidents," said Dennis Ross, the former Middle East envoy for Presidents George H.W. Bush and Clinton. "He wasn't just up to the level, he was superior."

When "Obama met with the Palestinians, some came up afterward and were saying that they were so impressed because here was Obama, who had the same facility, the same grasp of the issues as Clinton did," said Mr. Ross, who accompanied the candidate. "And Clinton had been president for a few years by the time he met with them."

Still, if Mr. Obama isn't easy to define for many Americans, that may be attributable to his unconventional life story. After his years in Hawaii and Indonesia, he attended Occidental University in Los Angeles before transferring to Columbia University in New York. He spent time in Chicago as a community organizer before heading off to Harvard Law School. He returned to Chicago to teach constitutional law and work as a civil-rights lawyer.

His varied experiences have given him a capacity to deal with many different types of people. He rose to be the first black president of the Harvard Law Review by winning over conservative classmates, then ran the journal in a way that appeased them and liberals alike.

There was "no question that he was a liberal, as were most editors of the Review and most students at Harvard Law at that time," said Brad Berenson, an associate White House counsel under President George W. Bush and a conservative lawyer in Washington who was an editor during Mr. Obama's tenure at the journal. "But he wasn't the kind who demonized conservatives or personalized politics."

That approach carried over later into his work in the Illinois legislature, where he ingratiated himself with Republican lawmakers. In Washington, Mr. Obama, who has one of the Senate's most liberal voting records, made alliances with Republicans such as Sens. Richard Lugar of Indiana and Tom Coburn of Oklahoma, one of the chamber's most conservative members.

In the nation's capital, Mr. Obama became known as a good listener.

Even in gatherings with some of the country's top business leaders, he "doesn't sit down to convince them how smart he is," said former Commerce Secretary Bill Daley, a supporter who attended a July meeting Mr. Obama held with advisers, including Robert Rubin and Warren Buffett.

"I've heard him say, `I just don't agree with you, I think there's a different way to look at this,' and then make the intelligent arguments that he needs to make," Mr. Daley said.

For his part, Mr. Obama said in an interview last month that "I'm a big believer in a management style that focuses on getting the job done and doesn't focus on personal ambitions or personal hang-ups."

If something doesn't work, "you cut your losses and move on," he said. "Learn from your mistakes and then try something else."

His generally temperate demeanor has earned him the nickname "No-Drama Obama." Said Mr. Berenson: "He always projected a kind of cool and a kind of reserve that masked the ambition that clearly had to be inside."

That ambition, though, has rarely been far from the surface.