Citing the senior Clinton adviser, The New York Times said Wednesday the New York senator is moving toward making the endorsement Saturday in New York.
Howard Wolfson, a chief Clinton campaign strategist, and other aides said Clinton would express support for Obama and party unity, and one Clinton adviser told the newspaper she would concede defeat and pledge to do whatever must be done to win the general election in November.
The development comes on a day when Democratic members of Congress urged Clinton to get out of the nomination race so the party can unite behind Obama, the presumptive presidential nominee, the adviser said.
Addressing supporters Tuesday night following primaries in South Dakota and Montana, Clinton said she would not make a decision on her campaign any time soon.
On the day Obama garnered enough commitments to surpass the 2,118 delegates needed to clinch the party's presidential nomination, Clinton said she was open to being Obama's running mate and pledged to work to unite the party after a long and bruising primary season.
She congratulated Obama for running an "extraordinary race" but conceded nothing, The Washington Post reported.
"I want the nearly 18 million Americans who voted for me to be respected, to be heard," she said to loud applause.