DENVER - When he claims the presidential nomination in front of 75,000 adoring supporters this week, Barack Obama will stand at the cusp of history.
The 47-year-old Hawaii native already is the first person of mixed racial parentage to crash the white man's club in American politics. That alone is a stunning achievement for someone who scarcely registered in the nation's consciousness when Democrats gathered for their last convention.
"The impossible has become possible," said Rep. Elijah E. Cummings, the Maryland co-chairman of Obama's campaign, who added that he never expected in his lifetime to see a person of color nominated for the country's highest office.
Having broken one barrier, Obama is reaching for something far greater. If the arc he set himself on years ago continues to rise, he'll be the nation's first African-American president, a tangible symbol of a profound shift under way in this country and a striking new image of U.S. leadership to the rest of the world.
Obama's crowning moment Thursday night in a pro football stadium at the foot of the Rocky Mountains is likely to provide an electric ending to this week's partisan festivities. But he can't savor it for long.
"There will be celebration for a moment, but they will realize clearly that there is one more step," said Cummings, a Baltimore Democrat. "To use a football analogy, they have to take this over the goal line."
That won't be as easy as some Obama supporters, expecting a landslide, once believed. The polls have tightened this summer, shaking the confidence of many on his side.
Obama is running as a change candidate in a year when voters seem eager to turn the page, but Democrats have questions: Why hasn't he pulled away from John McCain? How much will racial prejudice influence the outcome? Can he convince voters that he feels their economic pain? Is he tough enough to close the deal in November?
"I don't think that's just about me," Obama said in a recent interview with Time magazine. "I think they are congenitally nervous because we lost a bunch of presidential elections where people felt that we should have won. But keep in mind that whatever concerns people have about me, my campaign in particular, we heard those all through the primaries."
In those primary contests, he toppled his party's reigning dynasty, the Clintons, after the longest, most expensive struggle ever. Repairing the intraparty breach and bringing Hillary holdouts into the fold is a central convention goal.
At the same time, there's the fight ahead. He's been unable to widen his tiny advantage in the polls over McCain, who gets the last word next week in the second of the back-to-back conventions.
Obama is facing, in McCain, perhaps the only Republican who can overcome deep voter pessimism about how things are going under George W. Bush's unpopular presidency. McCain has been on the national scene for a long time, and his campaign is running commercials that promote him as "the original maverick."
Obama, by contrast, is a newcomer, more aloof by nature than his outgoing ticket mate, Joe Biden, and a puzzle to many voters. That makes the next several days particularly important, strategists said.
It's his last, best chance to exercise total control over the messages, both verbal and visual, that will bombard the nation's voters. Tonight will largely be devoted to telling the story of Obama's life, which he used to his advantage in the primaries.
He reverted to autobiography in an effort to quiet a storm of controversy over remarks by his former minister, which had raised questions about whether Obama was out of the mainstream of American thinking. In a Philadelphia speech back in March, he traced the broad outlines.
"I am the son of a black man from Kenya and a white woman from Kansas," Obama said. "I have brothers, sisters, nieces, nephews, uncles and cousins, of every race and every hue, scattered across three continents."
He described his wife, Michelle, who will address the delegates tonight along with his half-sister, as "a black American who carries within her the blood of slaves and slave owners - an inheritance we pass on to our two precious daughters."
Obama himself has emphasized, in recent campaign appearances, his late mother's struggle in raising two children as a single parent, resorting to food stamps at one point to make ends meet. It's an effort to counter McCain's portrayal of him as an elitist, as well as a way to reach out to women voters. But strategists and independent analysts said that general election voters aren't so interested in the uplifting rhetoric that inspired Democratic primary voters. Obama's convention task is more down-to-earth.