Sen. Hillary Clinton, however, will not concede defeat tonight or end her attempt to make history by becoming the first female presidential nominee.
Clinton campaign chairman Terry McAuliffe rushed onto CNN this morning to deny an Associated Press report that Clinton would concede tonight after polls closed in the final two Democratic primaries in Montana and South Dakota.
McAuliffe called the report "100 percent incorrect."
"The race goes on," McAuliffe insisted.
Moments later the Clinton campaign issued a statement saying, "The AP story is incorrect. Senator Clinton will not concede the nomination this evening."
"She is in this race until we have a nominee," a senior Clinton official told ABC News, "We do not expect there to be one tonight."
Obama is expected to speak shortly after 10 p.m. ET while the votes are still being counted in Montana and South Dakota, ending a grueling campaign that has set records for voter turnout all across the country.
Clinton, D-N.Y., will spend primary day at her New York home in Chappaqua and plans to give a speech in New York Tuesday night -- the first time the candidate has not spent a primary night in a contested or about to be contested state.
The Clinton camp insists she will not concede the nomination in that speech, believing Obama will not reach the delegate number -- 2,118 -- needed to clinch the nomination. The candidate and her advisers are trying to decide whether to keep fighting, drop out or simply suspend her campaign.
Obama Looks to General, Clinton Contemplates Next Move
Obama was almost giddy with exhaustion, relief and exhiliration as he showed off a new campaign bus Monday with his name plastered on the side.
Asked if he's emotional about being on the precipice of the nomination, he said to ask him later in the day.
Obama and Clinton have battled for six months with neither able to deliver a knockout blow, but Obama has amassed a majority of the party's pledged delegates and his tally continues to grow as superdelegates, eager to unite behind a candidate, announce their support.
Stephanopoulos said that party leaders like House Speaker Nancy Pelosi and Senate Majority Leader Harry Reid will come out Wednesday and press the remaining superdelegates to make up their minds by Friday to bring the race to a close.
By this morning, Obama needed a little more than 30 additional delegates to reach the magic number of 2,118 to clinch the nomination. Clinton needs nearly 200.
House Majority Whip Rep. James Clyburn of South Carolina went public with his support for Obama, calling the Illinois senator "our nation's best hope for much needed change."
Four other superdelegates quickly followed Clyburn's lead and others, including many members of Congress, are simply waiting, out of respect for Clinton, for the primaries to end to break their silence and endorse Obama.
Healing Democratic Wounds
The next crucial step for the Democrats will be to smooth over the differences between the Obama and Clinton camps so the winner can take on Sen. John McCain, R-Ariz., the presumptive Republican nominee, in the fall.
Obama said Monday that when he talked to Clinton after her Sunday win in Puerto Rico's primary he told her that once the dust had settled, he was looking forward to meeting her at a time and place of her choosing.
There were some bittersweet moments for the Clintons as the campaign came to a close.
An ABC News camera caught Hillary and Bill Clinton in a tender moment on the plane home Monday night that seemed to sum up the grueling campaign. The former president tenderly put his hand on his wife's cheek and then her shoulder, and she responded with a yawn that appeared to drain her whole body.
In what may be one of the last speeches of her campaign, Clinton told a crowd of supporters in Sioux Falls, S.D., "I could not be standing here asking for your support tomorrow without having been the beneficiary of the sacrifice of my family, but of generations of Americans before. … I want to be sure that that dream stays alive."
Bill Clinton was wistful, saying, "This may be the last day I'm ever involved in a campaign of this kind."
But at another point he lashed out bitterly at what he called "the national media's attempt to nail Hillary for Obama... It's just the most biased press coverage in history. … That's what they do -- he gets other people to slime her."
Some Clinton supporters aren't ready to call it quits.
The group Women Count, a PAC of female Clinton supporters, has a new ad running in The Hill and Roll Call newspapers aimed squarely at superdelegates.
"You're still not listening. Our votes are our voices," read the top headline. There is a photo of Clinton and bullet points with her arguments about winning the popular vote, and being the stronger candidate in a general election.
At the bottom of the page it reads: "Superdelegates, look at the facts. The voters have spoken. And remember this is not about you. It's about us."
Stephanopoulos said there is still some pressure for a "dream ticket" of Obama and Clinton, but he reported there is a "lot of resistance in the Obama campaign."