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No one believes in trickle-down economics. As GP said, it's a pejorative straw man invented by its opponents.
Except the folks that implement it:
> The cuts were based on model legislation published by the conservative American Legislative Exchange Council (ALEC),[8][9][10][11] supported by supply-side economist Arthur Laffer,[12] anti-tax leader Grover Norquist,[13] and the influential industrialists Charles and David Koch.[14][15]
* https://en.wikipedia.org/wiki/Kansas_experiment
The term was invented by its opponents, but it was a label that was put on actual policies, like Hoover's:
> In 1932, humorist and social commentator Will Rogers wrote a column criticizing Herbert Hoover's policies and approach to The Great Depression and stated: "The money was all appropriated for the top in the hopes that it would trickle down to the needy. Mr. Hoover was an engineer. He knew that water trickles down. Put it uphill and let it go and it will reach the driest little spot."[11] In 2007, political commentator William J. Bennett credited Rogers for coining the term "trickle down" and observed its persistent use throughout the decades since.[12]
And Reagan's:
> Ronald Reagan launched his 1980 campaign for the presidency on a platform advocating for supply-side economics. During the 1980 Republican Party presidential primaries, George H. W. Bush had derided Reagan's economic approach as "voodoo economics".[16] Following Reagan's election, the "trickle-down" reached wide circulation with the publication of "The Education of David Stockman", a December 1981 interview of Reagan's incoming Office of Management and Budget director David Stockman, in the magazine Atlantic Monthly. In the interview, Stockman expressed doubts about supply side economics, telling journalist William Greider that the Kemp–Roth Tax Cut was a way to rebrand a tax cut for the top income bracket to make it easier to pass into law.[17] Stockman said that "It's kind of hard to sell 'trickle down,' so the supply-side formula was the only way to get a tax policy that was really 'trickle down.' Supply-side is 'trickle-down' theory."[17][18][19] Reagan administration officials including Michael Deaver wanted Stockman to be fired in response to his comments, but he was ultimately kept on in exchange for a private apology.[20]
* https://en.wikipedia.org/wiki/Trickle-down_economics
It is pejorative, but it is not attacking a straw man, but actual policies that right-wing folks want implemented (and have).