The presumptive Democratic nominee is expected to win an additional number of delegates from today's contests and further superdelegate endorsements Tuesday.
Making history by becoming the nation's first African-American presidential nominee, Obama, D-Ill., emerges victorious from one of the longest and most closely fought Democratic nomination fights in recent history.
"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 cheering supporters at an arena in St. Paul, Minnesota.
Clinton Refuses to Concede Race
However Obama's closest Democratic rival Sen. Hillary Rodham Clinton refused to concede the race Tuesday night.
"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 August.
Clinton's campaign has indicated her willingness to challenge a Democratic Party ruling on Michigan and Florida's disputed delegates all the way to the Denver convention - a drastic option that would delay the party uniting behind Obama.
However Clinton may be trying to leverage her support for a place on the ticket as Obama's running mate.
Speaking on a conference call to fellow New York lawmakers Tuesday, Clinton said she is "open" to being Obama's vice presidential candidate if he asks, a source on the call told ABC News' Rick Klein.
Late Tuesday night, Lanny Davis, a long-time friend of Sen. Clinton's, circulated a petition asking Obama to choose Clinton as his running mate.
Obama Faces Divided Party
The win is a huge accomplishment for Obama, 46, a first-term U.S. senator who would be among the youngest presidents in U.S. history 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 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.
In an attempt to heal a divided Democratic party, Obama offered effusive praise for Clinton's historic bid to be the party's first woman Democratic presidential nominee.
"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.
Obama, McCain Launch Campaign Fight
Standing on the same stage where Sen. John McCain will accept the Republican party's presidential nomination in September, Obama quickly pivoted to the general election, taking a swipe at his Republican opponent.
"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 is expected to say.
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 is expected to say. ""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."
Launching the general election fight full-throttle, McCain fired a shot at Obama in a nationally televised speech made before Obama had declared victory.
"I have a few years on my opponent, so I am surprised that a young man has bought in to so many failed ideas," McCain, 71, told supporters Tuesday night in Kenner, Louisiana.
"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."
Calling Clinton "my friend," McCain heaped praise on the former first lady, suggesting "pundits" and "Democratic "Party elders" unfairly crowned Obama the Democratic nominee.
"The media often overlooked how compassionately she spoke to the concerns and dreams of millions of Americans, and she deserves a lot more appreciation than she sometimes received," McCain said. "As the father of three daughters, I owe her a debt for inspiring millions of women to believe there is no opportunity in this great country beyond their reach."
Clinton 'Open' to VP Slot
As the last day of a grueling, five-month Democratic primary battle fight came to a close, Clinton watched as superdelegates flocked to her opponent and told fellow New York lawmakers that she is open to being Obama's vice presidential candidate if he asks.
A year ago, the former first lady led every Democratic presidential candidate in the polls and was considered the party frontrunner with big-money Democratic donors, the support of the Democratic establishment, and the backing of her husband, former President Bill Clinton.
But today Clinton, trailing Obama in delegates and her campaign deeply in debt, spent much of the afternoon calling major donors and supporters from her home in Chappaqua, N.Y., in a last-ditch effort to gauge her support.
Obama Wins Delegate Race
With only 31 pledged delegates at stake in Tuesday's final primary contests in Montana and South Dakota, the Obama campaign pressed uncommitted superdelegates Tuesday to announce their support before the polls close tonight to allow him to emerge as the party's nominee without the appearance of it all coming down to the superdelegates.
He saw an avalanche of superdelegates come his way in the last two days before the final primary contests, including former President Jimmy Carter, and renowned civil-rights leader and House Majority Whip Rep. James Clyburn, D-S.D.
Of the 796 Democratic superdelegates -- party officials, members of Congress and state party leaders free to back any candidate -- less than 200 were still waiting to declare their support for either candidate when the day began.
"We've known for the last couple of months that even though Obama emerged as the front-runner, he was not going to be able to secure the nomination without the support of superdelegates," said Nathan Gonzales, political editor of the Rothenberg Political Report.
Superdelegates Make Impact
Trailing Obama in delegates and her campaign deeply in debt, Clinton has faced pressure to drop out of the race but refused to leave until the last primary states voted.
This afternoon Democratic Majority Leader Harry Reid of Nevada told reporters that he encouraged uncommitted superdelegates in the Senate to hold off on endorsing immediately and to wait for results of tonight's primaries.
"Sen. Clinton needs to be left alone to get through the primary process and let it run it's course," Reid said.
Candidates Woo Undecided Superdelegates
While their phones have been burning up for months with calls from the candidates and former President Bill Clinton, many superdelegates were uncomfortable with their roles as potential kingmakers.
"For senators, you've got a race here between two of their colleagues, and at least one of them is coming back to the Senate, so I think their reluctance to pick a side is to maintain good relations," said Jennifer Duffy of the Cook Political Report.
Clyburn Tuesday urged fellow superdelegates to get off the fence.
"Sen. Obama brings a new vision for our future and new voters to our cause. He has created levels of energy and excitement that I have not witnessed since the 1960s," read Clyburn's statement released Tuesday.
Many prominent Democrats, including House Speaker Nancy Pelosi have urged fellow Democrats to vote the way their state does.
Role of Superdelegates Scrutinized
The role of superdelegates may be given more scrutiny after the grueling, neck-and-neck primary battle between Obama and Clinton.
The Democratic Party decided more than three decades ago that party leaders and former Democratic politicians should become the ultimate deciders in a tight race.
After the insurgent outsider campaigns of George McGovern and Jimmy Carter won the Democratic Party nominations in 1972 and 1976, many party officials felt the need to have a greater role in the nominating process.
Apart from convincing big numbers of undecided superdelegates to back her presidential bid, the only option left to Clinton is to push her fight to the Democratic convention in late August, a move opposed by party leaders eager to stop the infighting and start fighting against McCain.
With her considerable support from voters, Clinton may also be trying to leverage herself to negotiate with Obama on various matters, including a potential spot on the ticket as the vice presidential candidate or influence Obama on policy issues like health care.
Asked Tuesday by a South Dakota radio station about his vice presidential choice, Obama said.
Of Clinton, he said, "She's run a magnificent race&I'm sure that we will have ample time to sit down and talk about bringing the party together and make sure that we are focused on November."
Entering the general election with aRepublican president with record-low approval ratings and polls suggesting over 80 percent of Americans believe the country is on the wrong track, Obama has a good shot at the White House.
However Obama emerges bruised from a bitter Democratic primary battle, and faces the daunting challenge 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.
With reporting by ABC News' David Wright, Jake Tapper, Karen Travers, Rick Klein, Kate Snow, Sunlen Miller, Karin Weinberg, and Eloise Harper.