Sen. Barack Obama, D-Ill., is expected to capture enough delegates in Tuesday's remaining two Democratic primaries to declare himself the Democratic Party's presidential contender and become the first African-American nominee of a major political party.
"He will declare victory tonight in a moment of history," ABC News' chief Washington correspondent George Stephanopoulos told "Good Morning America."
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.
"Obama will walk onstage tonight as the nominee," an aide to the senator told ABC News, and his speech will reflect that new status. The aide said Obama's comments will center on "a new phase, a new beginning" and make an appeal for Democratic Party unity.
The Democratic front-runner kept up his voting day tradition of playing basketball, and when he emerged from his game still in workout clothes, he told ABC News, "I'm doing fine but I thought this was off limits, basketball. I'm doing all right."
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 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.