Edward J. McCaffery is Robert C. Packard trustee chair in law and a professor of law, economics and political science at the University of Southern California. He is the author of "Fair Not Flat: How to Make the Tax System Better and Simpler" and founder of the People's Tax Page. The opinions expressed in this commentary are his own. View more opinion at CNN.

(CNN) Unlike his rivals, New York City Mayor Michael Bloomberg is running a "self-financed" campaign for president. The 78-year-old entrepreneur with an estimated net worth of around $62 billion has pledged to spend "whatever it takes" out of his fortune to defeat President Donald Trump.

Edward J. McCaffery

But in a very meaningful sense, Mayor Bloomberg is playing with what in the long run would be house money. The public is effectively picking up at least 40% of the tab for Bloomberg's massively financed run. The reason is the existence of the US estate or "death" tax. The simple fact of the matter is that, as an aging billionaire many times over, Mayor Bloomberg sees the clock ticking on his ability to play Estate Planning 101: the simple advice to spend it all and die broke

Let me explain.

If Bloomberg were to meet his maker still holding onto his full $62 billion, the federal government would take 40% of that sum, nearly $25 billion, in the estate tax -- even after the tax's dramatic weakening in the 2017 Trump tax cut. Of course, Mr. Bloomberg could try to dodge the estate tax through tax manuevers and get billions to his personal heirs, tax-free, as his fellow billionaire Sheldon Adelson and many others have reportedly done.

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Or he could join with other billionaires like Bill Gates and Warren Buffett in pledging to give a majority of his wealth to charity. But there is nothing illegal about choosing Door No. 3, and going on a grand binge , as Jeff Bezos is doing to the tune of $1 billion a year on his pet project of outer space technologies. Any dollar spent today is a dollar not taxed tomorrow under the estate tax.