But his most difficult challenge might be to wage a campaign that avoids tarnishing his image as an uncompromised alternative to the stale and polarized politics of the past.

"He is fresh. He is not part of the problem," observed Stephen Wayne, an expert on presidential politics at Georgetown University. "His message is that there's hope, and he is the personification of what's good with America. Look at what America did for him, and now he wants to return the favor.

"That's a very, very appealing image. There are a lot of Republicans who are very, very anti-(Sen. Hillary) Clinton. I don't know anybody who is anti-Obama. So don't give them a reason to be mad at you."

But safeguarding that trademark might prove difficult, given what is shaping up as a competitive struggle for the Democratic nomination followed by a showdown with the Republican Party for control of the White House.

Voters, let alone his competitors, will demand that Obama, a neophyte on the national stage, prove his readiness for the presidency. That will require moving beyond gauzy bromides about the promise of America to laying out a specific platform for dealing with a laundry list of problems, from Iraq to health care, education, global warming and energy.

"Obviously, you have to have a bigger message, as (Obama) has articulated about hope and moving the country forward," observed Steve Elmendorf, deputy campaign manager for John Kerry's unsuccessful presidential campaign in 2004. "But at the end of the day, you still have to have policies. I don't care how big your message is, you've got to have something specific to say about things."

Meeting this demand will pose the temptation to tailor his proposals to appeal to specific interest groups. Past presidential campaigns are replete with candidates who pandered to their party's most fervent supporters in the primaries, only to falter in trying to claim the center in the general election.

"If he is going to have a shot at this - above and beyond his charisma," said John Zogby, an independent pollster, "he has to represent something different, which is to go beyond being the blue state candidate for blue states only" - a reference to Democratic states.

How can Obama pull this off?

John Pitney, former acting director of research for the Republican National Committee and now a political science professor at Claremont McKenna College in California, pointed to President Bill Clinton's willingness in 1992 to defy liberals in winning the Democratic nomination. By supporting capital punishment, free trade with Mexico and welfare reform, Clinton preserved his credibility sufficiently to win the general election.

"If Obama wants to reach out to Republicans and independents (for a general election campaign), he needs to find an issue that shows he is not simply a party-line Democrat," Pitney said.

Obama's immediate itinerary following his announcement today calls for visits to Iowa and New Hampshire, two early-voting states where he will have to meet what might well be sky-high expectations for his candidacy.

As a practical matter, experts say, Obama might need to finish a strong second in the curtain-raising Iowa caucuses, do equally well in the Nevada caucuses that follow, and then win the first-in-the-nation New Hampshire primary. Failure to meet those benchmarks could cause money and media attention to dry up.

Next is the primary in South Carolina, which could develop into a showdown between Obama and Clinton over the black vote.

All that might be a prelude to what could be a titanic, coast-to-coast battle on Feb. 5, 2008, with possible primaries in such huge battlegrounds as California, Florida, Illinois and New Jersey as well as a number of smaller states. That would be a blitzkrieg across a complex political chessboard, requiring a deep and well-deployed organization, a huge bankroll and a high degree of name recognition.

If he gets that far, most analysts think Obama, with his star power, could be well positioned to win.

But first he has to survive Iowa, where Barbara Trish, a political scientist at Grinnell College, observed, "There is a threshold of organization you have to meet. If you don't, you can't be competitive.

"The excitement he brings to the race is going to help. It isn't enough, but it will turn some heads. He will draw in some activists who wouldn't otherwise be involved at this stage."

In New Hampshire, Dick Bennett, a pollster who focuses on the Granite State, said the contest is shaping up as a battle between "two rock stars" - Clinton and Obama.

"(Obama) may be able to pull it off, but he is in the middle of the pool right now," Bennett said.

"He dove in, and now he has to answer the questions. None of this, 'Let's all feel good together because I'm in the race.' It's time for, 'OK, what are you going to do for me now?'"