Donald Trump christens with champagne a new variety of rose that was named after him in a ceremony in Quito, Ecuador, on May 31, 2004. Reuters Democrats spent four days trashing GOP presidential nominee Donald Trump at their convention, but it seems that Trump has taken special issue with one speaker in particular: billionaire entrepreneur and former New York City Mayor Mike Bloomberg.

After Bloomberg questioned Trump's business acumen and called him a con man during his speech on Wednesday, Trump started referring to Bloomberg as "Little" Michael Bloomberg, reusing a moniker once reserved for Florida Sen. Marco Rubio.

Trump also made a clear reference to Bloomberg at a rally in Iowa on Thursday: "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."

Bloomberg is not a Democrat. Nor is he running for office. But he gets the threat of physical violence and a fun nickname because someone like Bloomberg offending Trump is especially offensive to Trump.

Any New Yorker can tell you why. It's because Trump has spent decades ignoring one of the cardinal rules of being publicly rich in this town: If you must flaunt your money, then you must also give it away in large sums.

If you don't, then you'll spend your life feeling like an outsider, building resentment against those who should be your set. And that's exactly what Trump has done.

America's Trump, New York City's Trump

While America has spent decades seeing Trump as a billionaire entrepreneur with a lux lifestyle, New York's elite have spent the same amount of time seeing him very differently — as a tacky sideshow with no regard for social norms.

No, this isn't Edith Wharton's New York City, but there are still rules. For one, if you want to attend everyone's fancy charity parties, then you've got to make a donation. If you don't make the donation, then prepare for everyone to stare daggers at you, to worry about your financial situation, to judge you.

If you don't give, then eventually people will invite you to fewer events. They'll consider you especially rude for going to the ones that you have been invited to out of politeness. That is when you are truly rejected by people who, ideally, since you're a "billionaire, too," should be your peers.

And that's when the gauche things you do as a rich person are suddenly inexcusable. People may see you at parties, but they do not really see you. They see a clown, a joke, a fraud.

This isn't something that started just this year, when Trump claimed to have donated $1 million to a veterans organization and was found to have done nothing of the sort. He eventually did.

Trump has a well-documented history of pledging to make donations and then reneging in this town, and in quiet whispers anyone on Wall Street will tell you that.

The media has looked into Trump's charity history, too. From The Washington Post (emphasis added):

"The Post contacted 167 charities searching for evidence of personal gifts from Trump in the period between 2008 and this May. The Post sought out charities that had some link to Trump, either because he had given them his foundation's money, appeared at their charity galas or praised them publicly.

"The search turned up just one donation in that period — a 2009 gift of between $5,000 and $9,999 to the Police Athletic League of New York City.

"In all, when the $1 million gift to veterans is added to his giving through the Donald J. Trump Foundation, Trump has given at least $3.8 million to charity since 2001. That is a significant sum, although not among billionaires. For example, hedge fund titan Stanley Druckenmiller, just behind Trump on Forbes's rankings of net worth, gave $120 million to his foundation in 2013 alone."

Former New York City Mayor Michael Bloomberg. REUTERS/Stephane Mahe

So enter Bloomberg, a man whose name is uttered with the utmost respect among New York City's elite. He was the city's mayor, and he's dedicated his vast fortune to Bloomberg Philanthropies, among other charitable causes.

Whether or not everyone agrees with his politics, his generosity is unquestionable. This is a man who writes $100 million checks for Bill and Melinda Gates' charitable work while doing his own. This is a man who has given away billions of his $37 billion fortune to those less fortunate.

This is in sharp contrast to Trump, who, according to The Washington Post, has given no money to his own foundation since 2008.

Hands

Over the years, Trump has yearned for Bloomberg's acceptance as he has been for the acceptance of others in Bloomberg's cohort. Again, he considers himself, as a fellow billionaire, part of their world. Back when Bloomberg was considering a run for president himself, Trump told CNN, "We used to be friends. I guess we're not friends anymore."

Oh, you thought you guys were friends?

Trump was never to be friends with the likes of Michael Bloomberg. That fact was stunningly clear to New Yorkers as early as 1990, when Spy Magazine — a now defunct publication started by a captain of New York society, Graydon Carter, now Vanity Fair editor — played a nasty trick on a bunch of New York City's wealthiest.

To find out how many of the city's rich actually acted quite poorly, Spy sent a bunch of checks in very small amounts — $1.11, $2, $0.16 — to dozens of people from Cher to author Kurt Vonnegut, from media mogul Rupert Murdoch to private-equity billionaire Henry Kravis.

Carter and his partner, Kurt Andersen, likely howled as they watched the checks being cashed. Only two people cashed the smallest, $0.13 check, though.

From Spy, which is still, and likely always will be, worth the read:

Spy Magazine. Spy Magazine, screenshot

Spy also famously called Trump a "short-fingered vulgarian," which he's never gotten over. Yes, this is why he talks about his hands all the time. It's because the people of New York City giggled about them about 30 years ago.

And New York City's elite have been quietly chortling about them ever since.

This understanding of Trump's relationship with those in his social class should also provide some clarity as to why Wall Street has given relatively no money to Trump's campaign. You see, it's like this: He never gave to their charity cases.

So they're not giving to his.