Bloomberg's Endorsement of Clinton Welcomed by New York Delegates

PHILADELPHIA -- One of the biggest speakers at the Democratic National Convention tonight hasn't been a member of the party for 15 years.

Former New York City Mayor Michael Bloomberg is due to endorse Hillary Clinton tonight, which comes as a surprise for many since he was at one point contemplating a presidential run himself.

Bloomberg's appearance on the Democratic stage is also something of a surprise since he removed himself from the party in 2001 and became a Republican, before switching to become an Independent.

For New York Democrats attending the convention, they see the move as an example of crossing the aisle as opposed to him coming back into the fold.

Ruben Diaz Jr., the Bronx Borough President, said that he and Bloomberg "didn't agree on everything" when Bloomberg was in office, but they were able to work together on several fronts and sees it as an important signal that a man who "understands corporate culture" is choosing to get behind Clinton as opposed to Donald Trump.

For New York Assemblyman Michael Blake, who is also attending the convention as a delegate, Bloomberg's endorsement means a good deal because Bloomberg clearly knows both candidates quite well.

"He had the choice of two New Yorkers -- neither of whom are members of his current party -- and he made the choice that he knows is best for the country," Blake told ABC News from the floor of the convention Tuesday night.

"Mayor Bloomberg is just another person who knows her well," he said. "Republicans, Independents and Democrats all realize she's the best choice."

While Bloomberg and Clinton are not particularly close, Bloomberg has made no secret he would like to see Trump lose the election.

Bloomberg very publicly considered running as a third-party candidate for president. But he penned an op-ed in March calling it "The Risk I Will Not Take."

In the op-ed, he didn't mention Clinton once, yet explained that his decision not to run was because "my candidacy could lead to the election of Donald Trump or Senator Ted Cruz. That is not a risk I can take in good conscience," he said of the two Republicans who were in the midst of their party's primary at that time.

Longtime Bloomberg adviser Stu Loeser told ABC News the endorsement is a sign of Bloomberg's dismay with Republican nominee and fellow billionaire Donald Trump.