These days Donald Trump and Mike Bloomberg can best be described as mortal enemies.

The former New York mayor’s campaign against the president is leveraging Bloomberg’s deep pockets to oust his fellow Manhattan billionaire. Bloomberg has hired hundreds of staffers and organizers across 30 states. Bloomberg’s ads are ubiquitous on TV and his campaign has reserved a 60-second spot during the Super Bowl with an estimated price tag of $10m.

The Trump campaign, in an illustration of the arms race a general election matchup between Bloomberg and Trump would be, announced its own $10m Super Bowl ad reservation after Bloomberg.

On Monday, Bloomberg and Trump squabbled on Twitter, with Trump dinging the former New York mayor on healthcare, calling him “Mini Mike”. In response Bloomberg corrected Trump’s false claim that his administration protected the pre-existing conditions provision of Obamacare.

And recently, the financial news website Marketplace published a scathing op-ed by Bloomberg with the headline “Trump has been great for people like me – but I’ll be great for you”.

But it wasn’t wasn’t always like this.

Bloomberg and Trump, both billionaires from New York, for years kept a cordial and even friendly relationship as they repeatedly ran into each other at charity events, parties and even one of former New York mayor Rudy Giuliani’s weddings.

Michael Bloomberg and Donald Trump at the Trump National Golf Course in Briarcliff Manor, New York, on 20 July 2007. Photograph: Patrick McMullan/Patrick McMullan via Getty Images

Along the way they swapped praise. At a 2007 charity golf club event, Trump said it was “my really terrific privilege to introduce a man that I think is one of the great mayors and will go down as one of the great mayors, if not the greatest, in New York City”. In 2013, Trump was asked if he thought Bloomberg is a good mayor. He responded: “Yes!” At a ribbon-cutting ceremony that same year Trump said Bloomberg had “been a great mayor”, adding: “I mean, this guy is fantastic.”

Trump has even praised Bloomberg’s past positions on gun control on Fox News’s Fox & Friends.

Bloomberg has also thrown Trump some kind words. At the same ribbon-cutting ceremony Bloomberg said: “If there is anybody who has changed this city, it is Donald Trump.” In 2004, Bloomberg appeared on an episode of The Apprentice. Trump at the time said he invited Bloomberg on to the show because he had “great respect for him”.

Yet even back then the identities they each fostered as famous billionaires were radically different. They ran in different social circles. Where Trump would go to a dinner party hosted by Jeffrey Epstein, Bloomberg would go to editor Tina Brown’s house.

“I’m sure Bloomberg has no gold toilets at his house,” said Rebecca Katz, a New York-based Democratic strategist. “It’s a different kind of money with less to prove.”

Congressman Peter King, a New York Republican, recalls Bloomberg once telling him that the only time he really interacted with Trump was at a charity golf tournament.

“They were in many ways from different worlds,” King said. He added that before Trump ascended to national office he had only met him a few times while he knew Bloomberg “very well”. Bloomberg and people in his orbit rarely mentioned Trump’s name, King said.

“It wasn’t like they were hugging or talking about ‘Hey, remember the great times we had’ – it was like two guys who knew each other but didn’t seem overly close, they didn’t seem overly hostile,” King recalled.

Donald Trump, Mike Bloomberg and Jared Kushner in New York, New York, on 14 March 2013. Photograph: Paul Bruinooge/Patrick McMullan via Getty Images

But those days are long gone, leaving the hostility thoroughly overt and the antagonism is only likely to escalate as the two pour more money and time into the 2020 presidential race. Some think their previous social proximity might even make their rivalry more intense.

King, who Bloomberg has raised money for and who backs Trump, predicted that a head-to-head matchup between the two candidates would involve “a huge amount of money, a huge amount of ego on both sides”.

“Each guy would think he’s smarter than the other,” King said, adding that each billionaire would be acting like he’s accomplished more than the other.

The relationship is also extra testy because of Bloomberg’s unique position within the field of over a dozen Democratic candidates vying to face Trump.

Bloomberg has seen his national poll numbers rise within the Democratic primary as he’s poured money into advertising for his campaign. Bloomberg is the only one who hails from the same state as Trump (although Vermont senator Bernie Sanders was born in Brooklyn) and whose background as an outsider billionaire who is still a household name within political circles is similar to Trump’s.

Recently, Trump has reportedly started to wonder how much money Bloomberg could spend to defeat him. And Bloomberg is airing ads promising not to copy Trump’s tweeting habits if elected president.

Bloomberg has toyed with running for president multiple times but Trump’s performance in the White House seems to be the breaking point that has finally tipped him into the race.

Aides to the former New York mayor like to point to recent statements Bloomberg has made lambasting the Republican president. As he’s opened campaign offices across the US Bloomberg has sprinkled his public remarks with apocalyptic warnings of Trump winning re-election.

At his newly opened campaign office in Tennessee, Bloomberg said: “Donald Trump is trying to pull this country apart, and if you want a future we just have to pull it together.” That same day in Philadelphia Bloomberg said pointedly: “Donald Trump does not know how to manage. He’s never been a businessperson. He’s a real estate developer, promoter.”

Trump has criticized Bloomberg as well. In early December, Trump mockingly tweeted that “Mini Mike Bloomberg has instructed his third rate news organization” to investigate “President Trump, only”.

The antagonism goes back through 2016. Bloomberg delivered a speech at the Democratic national convention that summer saying he wasn’t there as “a member of any party” but to urge voters to help elect Hillary Clinton and defeat “a dangerous demagogue”. Trump around that time tweeted: “Little Michael Michael Bloomberg, who never had the guts to run for president, knows nothing about me. His last term as Mayor was a disaster!”

Michael Bloomberg with Donald Trump and Melania Knauss in Washington DC, on 28 April 2001. Photograph: Newscom/Alamy Stock Photo

At the beginning of 2019, the Washington Post chronicled the evolution of Trump and Bloomberg’s interactions. At the time Trump told the newspaper that he and Bloomberg used to like each other but the relationship “went strangely haywire once I ran for office”.

Bloomberg, Trump said, didn’t care about his political positions before he ran for the presidency.

“I’m for guns, he’s against guns,” Trump said. Though Trump, in the past, had praised Bloomberg’s positions on guns.

For the same article, Bloomberg told the paper that his “objection to Donald Trump is the way he’s filling his current role, in terms of representing the country, in terms of representing the public. There’s an attitude, and a style, and a lack of civility that I think is bad for the country and I find offensive.”