Hillary Clinton will end her campaign to be the next US president on Saturday, her campaign has said.
The development follows the results of the final primaries, leaving Mr Obama on 2,158 delegates to Ms Clinton's 1,926 according to CNN.
With the victory target set at 2,118 the only way Ms Clinton could win is by persuading superdelegates she stands a better chance of victory in the autumn than Mr Obama.
After initial defiance following her win in South Dakota, however, she is now expected to admit defeat.
"Senator Clinton will be hosting an event in Washington, DC to thank her supporters and express her support for Senator Obama and party unity," a statement issued by the Clinton campaign said.
"This event will be held on Saturday to accommodate more of Senator Clinton's supporters who want to attend."
The pair both addressed the American Israel Public Affairs Committee (Aipac) yesterday and, reportedly, briefly spoke to each other after the event.
Speculation is now growing over whether Mr Obama will chose Ms Clinton as his running mate against the Republicans' John McCain in the general election.
Analysts say she would prove popular with blue-collar workers and Hispanics, but has already proved a divisive figure in the past. She has made clear she is prepared to run on the same ticket as him.