NEW YORK (CBSNewYork/AP) — The battle of the billionaires is on.

Donald Trump is firing back at Michael Bloomberg after the former New York City mayor spoke out against the Republican presidential nominee earlier this week at the Democratic National Convention.

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Bloomberg, a previous Democrat who served as mayor as a Republican before turning independent, called Trump “risky, reckless and radical.”

“I’m a New Yorker, and New Yorkers know a con when we see one,” Bloomberg said, adding it’s time for Americans to unite to “defeat a dangerous demagogue.”

Trump responded Friday on Twitter, saying “Bloomberg never had the guts to run for president.”

The DNC featured no shortage of harsh attacks on Trump, CBS2 reported. On Thursday, he said he’d been been thinking of striking back.

“I wanted to hit those speakers so hard,” Trump said. “I was going to hit one guy in particular, a very little guy. I was going to hit this guy so hard his head would spin, he wouldn’t know what the hell happened. He came out of nowhere to make deals with me. ‘Would you help me with this? Would you make this deal and solve the problem?’ I solved the problem, I do a great job.”

Trump did not specify who he was talking about, but some have speculated that it’s Bloomberg.

Now a political independent, Bloomberg considered making a third-party run for president this year before opting against a campaign, expressing worry he would siphon away votes from Democratic nominee Hillary Clinton and inadvertently help elect Trump.

A three-term mayor who left office in 2013, Bloomberg has been sharply critical of Trump. He wrote in March that the real estate mogul has run “the most divisive and demagogic presidential campaign I can remember, preying on people’s prejudices and fears.”

While in office, Bloomberg had a cordial relationship with Clinton, who, as a senator from New York, was involved in the city’s post-9/11 rebuilding effort. He did, too, with Trump, who he knew from New York’s glitzy social circuit and from dealings with him as a developer.

But Trump’s hardline approach to immigration alienated Bloomberg, who often makes the case an open immigration policy is needed to keep the nation’s economy growing.

Bloomberg has also become arguably the nation’s leading gun-control advocate, spending millions of his own fortune to finance candidates and groups that call for the restriction of firearms. Trump, meanwhile, has courted the support of the National Rifle Association, a pro-gun lobbying group that frequently criticizes Bloomberg.

Bloomberg, worth an estimated $47 billion, is 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. Largely a social liberal but a fiscal conservative, he served for 12 years, overseeing a gilded age in the nation’s largest city even as the gap between its rich and poor grew.

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