Sen. Barack Obama of Illinois wrapped up the Democratic presidential nomination last night, positioning him to become the first person of color in American history to lead a major party's quest for the White House.

By the time the polls closed in the final primary states of South Dakota and Montana, Obama had secured the backing of more than enough superdelegates to assure his triumph over Hillary Rodham Clinton, whose candidacy once seemed nothing short of inevitable.

Earlier in the day, Clinton said for the first time that she'd be "open" to running for vice president, increasing the possible downside for Obama should he decide not to pick her.

In his new role as presumptive nominee, Obama embarked last night on a five-month, general-election campaign against John McCain, using as his launching pad the arena in St. Paul., Minn., where the Republicans are to nominate the Arizona senator come September.

After proclaiming that "I will be the Democratic nominee for president of the United States," Obama pledged to engage his opponent in a high-minded debate.

"What you won't hear from this campaign or this party is the kid of politics that uses religion as a wedge, and patriotism as a bludgeon," he told the cheering crowd. ". . . Because we may call ourselves Democrats and Republicans, but we are Americans first. We are always Americans first."

Clinton, who won in South Dakota, did not concede defeat, saying at her own rally in New York that she'd be "making no decisions tonight."

She praised Obama, who won Montana, and spoke of party unity. But she also made the familiar arguments for her candidacy, highlighting her strength in key states and in the overall popular vote.

"I understand that a lot of people are asking, 'What does Hillary want?'" she said. ". . . I want the 18 million people who voted for me to be respected, to be heard and no longer be invisible."

Obama, who burst on the scene with his stunning speech at the Democratic National Convention four years ago, now has to figure out how to deal with Clinton and her disgruntled supporters. He tossed numerous oral bouquets in her direction last night.

"Senator Hillary Clinton has made history in this campaign not just because she's a woman who has done what no woman has done before," he said, "but because she's a leader who inspires millions of Americans with her strength, her courage, and her commitment to the causes that brought us here tonight."

He must also prepare for a November election that figures to test the attitudes of the American electorate toward the war in Iraq, the performance of the Bush administration, the state of the economy, the way to deal with foreign adversaries, and the role of race in American life.

No one knows whether the American people are ready to put a black man in the White House - this black man in particular. Even a few years ago, few people could have imagined that such a question would be front and center in the politics of 2008.

Obama is the nation's sole black senator, its fifth ever; there are currently two black governors, Deval Patrick of Massachusetts and David Paterson of New York.

Several other factors beyond the stark issue-differences between the two candiates will be at the heart of a contest that current polls suggest looks extremely close. Among those factors are age and experience.

By Election Day, Nov. 4, McCain, the former Vietnam prisoner of war will be 72 years old, having served nearly four full terms in the U.S. Senate after a distinguished career in the Navy.

Obama, who was a state legislator at the time of the last presidential election, will be 47, with less than four years in the Senate on his resume.

Speaking in Louisiana last night, in an address in which he said he represented the "right change for America," McCain drew the contrast.

"The American people didn't get to know me yesterday, as they are just getting to know Senator Obama," he said. "They know I have a long record of bipartisan problem solving. They've seen me put our country before any President, before any party, before any special interest, before my own interest."

A former community organizer who taught constitutional law at the University of Chicago, Obama won the Democratic nomination by voicing the desire of many for a break with politics as usual, winning broad support among younger people, highly educated voters and African Americans.

He won by raising more money than Clinton and by out-organizing her. He won by talking about hope, about change, and about bringing people together, as he did again last night.

And he won by telling his own story.

The son of a black father from Kenya and a white mother from Kansas, he grew up in Indonesia and Hawaii and was educated at some of the nation's most prestigious universities before making a home for himself in Chicago.

But as the primaries went on this spring, Obama did not wear all that well with voters. He inherits a party split virtually down the middle.

While there was little disagreement on the issues between him and Clinton, many of her supporters showed a distinct lack of enthusiasm for him, even as his nomination became a foregone conclusion.

Clinton's female backers, in particular, expressed resentment over what they saw as their candidate's sexist treatment by some in the news media - and disappointment at losing a historic opportunity to put a woman in the Oval Office.

In that regard, attention will be paid to Obama's treatment of the former first lady and how she figures into his choice of a running mate.

Her supporters believe that she could help him in Ohio and Pennsylvania, where she defeated him handily in the primaries. She might also be able to help with constituencies she won throughout the season - rural voters, Catholics, seniors, Latinos, women, and white members of the working class.

For Obama, the day began with an endorsement from Rep. James Clyburn (D., S.C.), the House majority whip, and one of the nation's most respected black lawmakers.

As the hours passed, the tide swelled, with other superdelegates, including former President Jimmy Carter, moving into the Obama camp.

Out of deference to Clinton, many undeclared members of Congress held off until the polls closed in the west - and the undeclared senators decided to wait until she makes her move. Rep. Bob Brady of Philadelphia delivered his backing to Obama early in the evening.

Eventually, the magic number, 2,118, was surpassed, and the nomination belonged to Obama. It won't become official until the night of Aug. 27, at the Democratic National Convention in Denver.

Contact senior writer Larry Eichel at 215-854-2415 or leichel@phillynews.com.