Longtime Bloomberg adviser Stu Loeser tells ABC News the former New York City mayor will speak Wednesday in Philadelphia and endorse the former secretary of state.
Another senior adviser to Bloomberg Howard Wolfson said in a statement: "As the nation's leading independent and a pragmatic business leader Mike (Bloomberg) has supported candidates from both sides of the aisle. This week in Philadelphia he will make a strong case that the clear choice in this election is Hillary Clinton."
Loeser said the endorsement is a sign of Bloomberg's dismay with Republican nominee and fellow billionaire Donald Trump. While Bloomberg and Clinton are not particularly close, Bloomberg has made no secret he would like to see Trump lose the election.
The endorsement comes as a surprise. Bloomberg is a former Democrat, but was elected mayor of New York City as a Republican in 2001 and later became a Republican. Bloomberg was considering his own run for the presidency this cycle and has been critical of Trump during this campaign, but decided in March that mounting an independent run could help Trump's path to the White House. He has been especially critical of Trump's immigration and Muslim ban policies.
Wednesday is the same night President Obama and Vice President Joe Biden are expected to speak. Bloomberg served as New York City’s mayor for 12 years and endorsed Obama’s re-election in 2012.