He has singlehandedly kneecapped the net worth of many billionaires.

When did this happen?
The collective wealth of U.S. billionaires increased dramatically after Trumps 2017 tax cut.
For example, the wealth of the richest 748 Americans rose by 77% an increase of $2.2 trillion since the law’s enactment.
By 2023, U.S. billionaires were worth $3.6 trillion more than before the law passed.
During Trump’s first term alone, U.S. billionaires gained $1 trillion in wealth, with the top 50 billionaires collecting 80% of that gain.

"The evidence shows that the top 1%, especially billionaires, became much richer under Trump, due to both direct tax cuts and broader economic trends that disproportionately benefited the wealthy.
The tax policy changes enacted during his administration significantly increased the wealth gap between the richest Americans and the rest of the population."

americansfortaxfairness.org - Billionaires trillion richer since trump gop tax law

econofact.org - Did trumps tax cuts and jobs act confer greater benefits on rich americans

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commondreams.org - The richest have never been richer as trump arrives to make them richer still