The Illinois senator wrapped up the grueling battle for the Democratic Party's nomination before the votes were even counted Tuesday in the last two Democratic primaries in Montana and South Dakota.
"This is our moment. This is our time," Obama, who got a thumbs-up from his wife Michelle before she left the podium, told 30,000 thrilled supporters in St. Paul, Minn., Tuesday night.
Obama surpassed the 2,118 delegates needed to clinch his party's nomination, ending a five-month-long slugfest with Sen. Hillary Clinton.
Obama, 46, went on to easily win Montana by a margin of 57 percent to Clinton's 42 percent. But Clinton snatched one last upset victory in South Dakota, with a 55-44 percent win.
Obama picked up enough delegates from those states to pad his margin of victory and a flood of superdelegates rallied to his side.
Clinton Mulls Future as Dems Search for Unity
Obama claimed the Democratic nomination in the same St. Paul arena in which Sen. John McCain will accept the Republican nomination in early September.
"Tonight we mark the end of one historic journey with the beginning of another - a journey that will bring a new and better day to America. Because of you, tonight I can stand before you and say that I will be the Democratic nominee for president of the United States," Obama told his cheering supporters.
He graciously praised Clinton, despite the sometimes bitter exchanges the two had during the campaign.
"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, 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," Obama said.
"Our party and our country are better off because of her, and I am a better candidate for having had the honor to compete" with her, he said.
The candidates played phone tag until they were finally able to speak just after midnight.
Obama reiterated his invitation for a conversation and Clinton agreed, but no specific plans were made for a time or place for such a meeting.
Sen. Barack Obama, D-Ill., has clinched the number of delegates needed to win the Democratic presidential nomination over Sen. Hillary Clinton, D-N.Y., which makes him the first black major party nominee. (AP Photo)Both Democrats are scheduled to be in Washington the day after the last primary contests, but there was no public schedule for them to meet.
Obama must now turn and prepare to take on McCain, but before he can do that, or even bask in the enormity of his historical accomplishment, he must find a way to make peace with Clinton and her many ardent supporters.
Clinton Refuses to Concede Race
In her final primary night speech, Clinton refused to concede the race.
"This has been a long campaign, and I will be making no decisions tonight," she told supporters in New York.
"In the coming days, I'll be consulting with supporters and party leaders to determine how to move forward with the best interests of our party and our country guiding my way," she said as supporters chanted "Denver, Denver!" pointing to the party's convention in late August.
She also asked the question that so many political observers have been wondering.
"I understand a lot of people are asking what does Hillary want? What does she want?" Clinton asked.
She went on to say she wanted her supporters and her issues to be respected, but earlier in the day during a conference call with financial backers she said she would be open to accepting the vice presidential spot with Obama.
And on Tuesday, Lanny Davis, a long-time friend of Clinton's, circulated a petition asking Obama to choose Clinton as his running mate.
Democratic leaders have moved to head off any more Clinton maneuvers that could delay or complicate Obama's ability to take the fight to McCain as the undisputed Democratic nominee.
A joint statement by Senate Majority Leader Harry Reid, House Speaker Nancy Pelosi, Democratic National Committee Chair Howard Dean and Democratic Governors Association Chair Gov. Joe Manchin urged all remaining uncommitted superdelegates to make their decision known by this Friday.
Clinton confidante James Carville told "Good Morning America" that an emtionally charged presidential campaign can't be turned off like "flipping a switch."
"It's going to take a couple days," said Carville, who added that Obama has handled the situation with Clinton in recent days "as well as he can."
McCain Targets Obama, Offers General Election Argument
McCain didn't wait long to engage his long-awaited general election rival.
"I have a few years on my opponent, so I am surprised that a young man has bought into so many failed ideas," McCain, 71, told supporters Tuesday night in Kenner, La.
"You will hear from my opponent's campaign in every speech, every interview, every press release, that I'm running for President Bush's third term," McCain said. "Why does Senator Obama believe it's so important to repeat that idea over and over again? Because he knows it's very difficult to get Americans to believe something they know is false.
"Both Senator Obama and I promise we will end Washington's stagnant, unproductive partisanship," McCain said. "But he hasn't been willing to make the tough calls, to challenge his party, to risk criticism from his supporters, to bring real change to Washington. I have."
Obama fired right back.
Standing on the same stage where McCain will accept the Republican Party's presidential nomination in September, Obama said, "John McCain has spent a lot of time talking about trips to Iraq in the last few weeks, but maybe if he spent some time taking trips to the cities and towns that have been hardest hit by this economy — cities in Michigan, and Ohio, and right here in Minnesota - he'd understand the kind of change that people are looking for," Obama said.
"Change is a foreign policy that doesn't begin and end with a war that should've never been authorized and never been waged," Obama said. "It's time to refocus our efforts on al Qaeda's leadership and Afghanistan, and rally the world against the common threats of the 21st century - terrorism and nuclear weapons; climate change and poverty; genocide and disease. That's what change is."
Entering the general election with a Republican president with record-low approval ratings and polls suggesting over 80 percent of Americans believe the country is on the wrong track, Obama has already begun attempting to link McCain and Bush.
But Obama emerges bruised from a bitter Democratic primary battle, and faces the daunting challenge of uniting the party.
The Democratic nomination fight exposed Obama's challenges in gaining support from white, blue-collar voters, Hispanics, women, and older voters who supported Clinton's candidacy in huge numbers.
Nevertheless, winning the Democratic nomination is a huge accomplishment for Obama, a first-term U.S. senator who would be among the youngest presidents in U.S. history - and its first African American - if he wins the White House.
With a popular campaign message of hope and change, he attracted huge crowds, celebrity endorsements, and record-breaking campaign contributions. His candidacy also inspired a record turnout by black voters, and enjoyed wide support from independents, liberals, young voters and high-income Democrats.
Early on, Obama cast his campaign as a rejection of old-style Washington politics, and painted Clinton as an incumbent.
Obama's victory was also a crushing defeat for Clinton.
A year ago, the former first lady led every Democratic presidential candidate in the polls and was considered the party front-runner with big-money Democratic donors, the support of the Democratic establishment, and the backing of her husband, former President Bill Clinton.
ABC News' David Wright, Jake Tapper, Karen Travers, Rick Klein, Kate Snow, Sunlen Miller, Karin Weinberg, Eloise Harper and James Gerber contributed to this report.