The billionaire, who has spent months mulling a third-party run that would have roiled this year's already extraordinarily unpredictable presidential campaign, made his decision official through an editorial posted on the Bloomberg View website.
Bloomberg, in ending his third and likely final flirtation with a White House run, wrote that a three-way race could lead to no one winning a majority of electoral votes, which would send the race to the Republican-controlled House of Representatives — and, therefore, to one of the GOP front-runners.
"That is not a risk I can take in good conscience," Bloomberg wrote.
Bloomberg was blistering in his critique of Trump, currently the GOP front-runner, saying the real estate mogul has run "the most divisive and demagogic presidential campaign I can remember, preying on people's prejudices and fears."
He was similarly critical of Cruz, saying the Texas senator's "pandering on immigration may lack Trump's rhetorical excess, but it is no less extreme."
Bloomberg acknowledged that he and Trump had been on "friendly terms" and that he had twice agreed to be on Trump's reality TV show "The Apprentice." But the former mayor said Trump's campaign "appeals to our worst impulses."
"We cannot 'make America great again' by turning our backs on the values that made us the world's greatest nation in the first place," Bloomberg wrote. "I love our country too much to play a role in electing a candidate who would weaken our unity and darken our future — and so I will not enter the race for president of the United States."
Bloomberg made only an oblique reference to Democrats Hillary Clinton and Bernie Sanders and did not endorse a candidate. His aides indicated that Bloomberg may at some point offer an endorsement and use his wealth to try to influence the race, but cautioned that no decisions had been made.
The former three-term mayor — who had indicated he would spend $1 billion of his own money on the campaign — had set a mid-March deadline for his team of advisers to assess the feasibility of mounting a run, believing that waiting longer would imperil his ability to complete the petition process needed to get on the ballots in all 50 states.
He had taken some initial steps, cutting a mock TV ad, preparing to open campaign offices in Texas and North Carolina — states with early ballot access deadlines — and having aides begin to vet possible vice presidential candidates, including Michael Mullen, the retired admiral and former head of the Joint Chiefs of Staff.
Aides to Bloomberg, the 74-year-old Democrat-turned-Republican-turned-Democrat-turned-independent, believed the dominance of Trump among Republicans and the rise of Sanders amid Democrats had opened a centrist lane for a non-ideological, pragmatic campaign.
Their own polling suggested Bloomberg had a viable path to the needed 270 electoral votes if Trump and Sanders were the nominees, though that projection included some remarkably optimistic predictions for Bloomberg, including winning Tennessee and Georgia and tying Sanders in the senator's home state of Vermont.
But as Clinton racked off a string of recent victories, Bloomberg — known largely outside New York for his crusades against guns and Big Soda, positions likely unpopular with Republicans nationwide — grew worried that he would siphon more support from her than Trump, ensuring that part of the mayor's carefully managed legacy would be that he helped give Trump the White House.
If Clinton won the Democratic nomination, Bloomberg's aides believed he had enough support to send the race to the House — but not to win there.
Clinton said Monday during a town hall in Detroit that she had "the greatest respect for Michael Bloomberg" and that she looked "forward to continuing to work with him."
Sanders, meanwhile, said it was Bloomberg's decision but he was concerned "on a broader scale" that only billionaires like the former mayor believe they can run for office.
Spokeswomen for Trump and Cruz did not immediately respond to requests for comment.
One of the richest people in the United States, estimated to be worth $38 billion, Bloomberg has previously toyed with presidential runs, but concluded ahead of the 2008 and 2012 campaigns he could not win.
The founder of the financial news and information provider Bloomberg LP, he was a political novice when he launched an unlikely bid for mayor in 2001.
He is largely a social liberal — he fought for same-sex marriage in New York and is pro-abortion rights — and implemented a number of health reforms in New York City, banning smoking in public places and instituting calorie counts on menus.
He has also become one of the nation's most vocal proponents of gun control, using his fortune to bankroll candidates across the country who clash with the National Rifle Association. But liberals have found fault with his cozy ties to Wall Street and his unquestioned support for the New York Police Department, which drove down crime during his tenure but engaged in tactics that a federal judge later ruled discriminated against minorities.
After leaving office, he returned to running his company, which posted $9 billion in revenue last year, and continues to run his foundation. He gave away more than $500 million in 2015.
___