Speaking on the third night of the Democratic National Convention in Philadelphia, Bloomberg slammed Trump's business record and called the Republican nominee a "dangerous demagogue" who must be stopped in an explicit appeal to independent voters across the country.
"When I enter the voting booth each time, I look at the candidate, not the party label," Bloomberg, who has had various political affiliations, said in his prime-time speech. "There are times when I disagree with Hillary Clinton. But let me tell you, whatever our disagreements may be, I've come here to say: We must put them aside for the good of our country. And we must unite around the candidate who can defeat a dangerous demagogue."
Bloomberg used his perspective as a businessman, New Yorker and former mayor to hit Trump from multiple angles.
"I'm a New Yorker, and New Yorkers know a con when we see one," he said to cheers from the audience.
"We've heard a lot of talk in this campaign about needing a leader who understands business. I couldn't agree more. I've built a business, and I didn't start it with a million-dollar check from my father," he said in a slam to the real estate mogul's background.
"Throughout his career, Trump has left behind a well-documented record of bankruptcies, thousands of lawsuits, angry shareholders and contractors who feel cheated, and disillusioned customers who feel ripped off," Bloomberg said. "Trump says he wants to run the nation like he's run his business. God help us."
Bloomberg, who became New York's mayor just months after the September 11, 2001, terrorist attacks, said he personally witnessed Clinton's work in public office.
"I saw how Hillary Clinton worked with Republicans in Washington to ensure that New York got the help it needed to recover and rebuild," he said. "Throughout her time in the Senate, we didn't always agree -- but Hillary Clinton always listened. And that's the kind of approach we need in Washington today, and it just has to start in the White House."
Bloomberg joins a long list of Democratic leaders and liberal activists making the case that Clinton is the most experienced candidate for the White House.
But Bloomberg's assault on Trump from the Democratic stage marks a somewhat novel approach from the Clinton campaign as it looks to expand the week's narrative that includes deep-blue pitches from Democratic giants like first lady Michelle Obama and former President Bill Clinton.
As he prepared to speak Wednesday night, advisers to Bloomberg said his goal was to reach independents, moderates and swing voters.
"As one of America's business leaders, Bloomberg has become increasingly concerned about what a Trump presidency would mean for our economy. He will lay out why voters cannot trust Trump to ensure our economic future," an adviser said.
Speeches throughout the week have been a mix of deeply emotional discussions highlighting the history of Clinton's nomination to a lengthy, personal talk from Bill Clinton that sought to soften her image.
Bloomberg's pitch is unusual because he is not a party stalwart -- he used to be a Democrat before becoming a Republican and then an independent.
Bloomberg first won the mayor's office as a Republican but has often stood with Democrats, focusing much of his efforts since leaving office on gun control and fighting climate change.
He considered an independent bid for the White House
himself, calling the national debate in February "banal." Bloomberg, who is worth much more than Trump by his own company's estimates, would have likely self-funded a bid but ultimately decided against it.
On Wednesday, Bloomberg said the choice for him in this election has become crystal clear.
"Let's elect a sane, competent person with international experience," he said to the crowd in the arena as well as Americans around the country.
"The bottom line is: Trump is a risky, reckless and radical choice and we can't afford to make that choice," Bloomberg said. "Now, I know Hillary Clinton is not flawless. No candidate is. But she is the right choice and the responsible choice in this election."