With Clinton and Obama running neck and neck, the Democratic campaign took on a decidedly negative tone.
Days before the New Hampshire primary, Bill Clinton questioned Obama's record on Iraq, saying it was wrong he was able to trumpet superior judgment on Iraq by claiming he had been against the war from the start.
"Give me a break. This whole thing is the biggest fairy tale I've ever seen," Bill Clinton said.
The former president also drew outrage when he compared Obama's South Carolina win to the Rev. Jesse Jackson's primary wins in the state in 1984 and 1988.
The comments created a furor, with some suggesting Clinton was trying to dismiss Obama and bring attention to his race.
During the media firestorm over the issue, Clinton told a Philadelphia radio station the Obama campaign was playing the race card.
"I think that they played the race card on me. And we now know, from memos from the campaign and everything, that they planned to do it all along," Clinton said in a telephone interview with WHYY.
Trailing the two front-runners, Edwards dropped his presidential bid days before the Super Tuesday sweep of primaries Feb 5, when 22 states held primaries and caucuses.
With Obama and Clinton locked in a tight, escalatingly bitter fight for delegates, the first-of-it's kind Feb. 5 Democratic primary contests did nothing to clarify the Democratic field when Obama and Clinton traded Super Tuesday victories.
Obama went on to win 12 straight primary contests in February, but lost contests to Clinton in the delegate-rich states of Ohio and Texas.
His failure to wrap up the nomination and secure the number of delegates needed to win left him open to attacks by the Clinton campaign, which said he wasn't experienced enough to be commander-in-chief and that she was the better candidate to win against McCain.
Obama and Clinton continued to trade primary victories throughout the campaign, exposing the Illinois senator's weakness in attracting white, blue-collar voters.
In March tapes surfaced of incendiary sermons by Obama's longtime pastor the Rev. Jeremiah Wright in which the pastor said, "God Damn America" for the way it treats blacks, and said the United States brought on the 9/11 attacks with its own "terrorism."
Obama gently distanced himself from Wright, now retired, who had been his pastor for more than 20 years, who had married Obama and his wife, Michelle, and baptized both of his daughters.
The controversy led Obama to make one of the most stirring speeches of his campaign on the issue of race. Standing in the Constitution Center in Philadelphia, Obama condemned Wright's remarks, and then offered a historical perspective, citing "the complexities of race in this country that we've never really worked through."
"Race is an issue that I believe this nation cannot afford to ignore right now. We would be making the same mistake that the Rev. Wright made in his offending sermons about America -- to simplify and stereotype and amplify the negative to the point that it distorts reality," Obama said.
After Wright reiterated some of his remarks during a speech at the National Press Club and in television interviews, Obama strongly denounced him. He later resigned from the Trinity United Church of Christ in Chicago after another pastor, Father Michael Pfleger mocked Clinton.