In just the last four days, the President has tweeted four times about the former New York City mayor and current candidate for the 2020 Democratic nomination.

"Mini Mike Bloomberg doesn't get on the Democrat Debate Stage because he doesn't want to - he is a terrible debater and speaker." Trump tweeted just after 8:30 a.m. on Friday morning. "If he did, he would go down in the polls even more (if that is possible.)" He followed that up with this 30 minutes later: "Mini Mike Bloomberg ads are purposely wrong - A vanity project for him to get into the game. Nobody in many years has done for the USA what I have done for the USA, including the greatest economy in history, rebuilding our military, biggest ever tax & regulation cuts, & 2nd A!"

The amount of attention Trump is paying Bloomberg is wholly outsized when compared to the former mayor's current standing in the race. Bloomberg is running fifth in Real Clear Politics average of national polls on the 2020 Democratic primary and is non-existent in the first four states to vote. (Bloomberg is effectively skipping those states due to his late entry into the race.)

So, what gives? Why is Trump so obsessed with Bloomberg? Four major reasons:

1) Bloomberg is very, very rich: According to Forbes , Bloomberg is the eighth richest person in America, with a net worth north of $53 billion. Trump, for his part, clocks in at 275th, with a net worth of just over $3 billion. Trump uses wealth as a measuring stick of success and importance. And he is all too aware that Bloomberg is significantly richer than him.

2) Bloomberg is spending his money: It's not just that Bloomberg is a lot richer than Trump. It's that, despite the fact that he hasn't been running for president long, Bloomberg is spending tons of money to boost his campaign. As of Friday, : It's not just that Bloomberg is a lot richer than Trump. It's that, despite the fact that he hasn't been running for president long, Bloomberg is spending tons of money to boost his campaign. As of Friday, Bloomberg has spent $245 million on TV ads , a figure that dwarfs what other candidates -- including fellow billionaire Tom Steyer -- have dropped on commercials. (Shortly after Bloomberg announced he was spending $11 million to run a single ad during the Super Bowl, the Trump campaign announced they would do the same.) According to Fox Business Network's Charlie Gasparino , Bloomberg is telling his backers in the financial sector he plans to spend $2 billion to beat Trump -- whether as the Democratic nominee or at the head of an outside entity in the general election. "Note to self: The ads hitting the President's health care record are working," tweeted Bloomberg adviser Howard Wolfson on Friday morning. "Run more."

3) Bloomberg is a New Yorker: Trump, although he no longer is an official resident of New York, still thinks of the the state -- and New York City, in particular -- as home. His demeanor, his approach to business and politics -- it's all very much rooted in being a New Yorker. That Bloomberg is not only on Trump's home turf but spent more than a decade as the city's mayor irks the President to no end.

4) Bloomberg is a businessman: Trump has made no secret of his disdain for politicians. And his admiration and love for successful businessmen. Bloomberg is, without question, a hugely successful businessman. And as I mentioned above, a very rich one. Which means Trump can't dismiss him as some loser lifetime politician.

Add up all those factors and you can see why the President is spending time picking on the height of someone who isn't a top-tier candidate for the nomination. (Sidebar: The President of the United States is picking on someone's height.) This isn't about Bloomberg's poll numbers. This is about Trump feeling threatened by someone who he can't dismiss as poorer or less successful than him. And since those are the only ways in which Trump measures himself against, well, everyone, he can't get Bloomberg out of his head.