With about 2,500 delegates awarded, Obama's lead over Sen. Hillary Rodham Clinton stood at 55 - 1,275 to 1,220.
Obama took the lead for the first time Tuesday, benefiting from victories in eight straight contests. He holds a 134-delegate lead among pledged delegates, those won in primaries and caucuses.
Clinton has kept it closer with more endorsements from superdelegates, the party and elected officials who automatically attend the national convention and can support whomever they choose, regardless of the outcome of the primaries.
Both candidates are more than halfway to the 2,025 delegates needed to secure the nomination.
Obama campaign manager David Plouffe said Clinton can't catch up without help from the superdelegates.
"We believe it's next to impossible for Senator Clinton to close that pledged delegate count," Plouffe said, referring to delegates won in primaries.
"The only way she could do it is winning most of the rest of the contests by 25 to 30 points and we see not a single contest on the calendar left where we would expect her to win by those kind of margins," he said in a conference call with reporters.
Clinton is pinning her hopes on delegate-rich Texas and Ohio; both have primaries March 4. Her chief strategist, Mark Penn, likened the race to difficult nomination battles won by Walter Mondale and Jimmy Carter.
"This is a competitive nomination race," Penn told reporters Wednesday. "It is not unusual historically for these races to seesaw back and forth."
Clinton's campaign spokesman, Howard Wolfson, said, "No one is going to get to 2,025 delegates without a considerable number of superdelegates."
Obama swept the primaries Tuesday in Maryland, Virginia and the District of Columbia, winning about twice as many delegates as Clinton - a tough task considering Democrats award delegates proportionally.
Twenty-one delegates remained to be awarded because of incomplete results.
Obama also picked up several endorsements Wednesday from superdelegates, including from Puerto Rican Gov. Anibal Acevedo Vila and David Wilhelm, who managed former President Clinton's 1992 campaign.
A breakdown of the race for Democratic delegates: Pledged delegates won in primaries and caucuses: Obama, 1,112; Clinton, 978.
Superdelegates: Obama, 163; Clinton, 242.
On the Republican side, Sen. John McCain captured 107 of the 113 delegates at stake Tuesday. The remaining six delegates were still undecided because of incomplete results in two Maryland congressional districts.
McCain leads the overall race for the nomination with 843 delegates, to 242 for former Arkansas Gov. Mike Huckabee.
It will take 1,191 delegates to secure the Republican nomination.
The AP tracks the delegate races by calculating the number of national convention delegates won by candidates in each presidential primary or caucus, based on state and national party rules, and by interviewing unpledged delegates to obtain their preferences.
Most primaries and some caucuses are binding, meaning delegates won by the candidates are pledged to support that candidate at the national conventions.
Political parties in some states, however, use multistep procedures to award national delegates. Typically, such states use local caucuses to elect delegates to state or congressional district conventions, where national delegates are selected. In these states, the AP uses the results from local caucuses to calculate the number of national delegates each candidate will win, if the candidate's level of support at the caucus doesn't change.
