During the 1980 presidential campaign, the Republicans were looking for a candidate to rob President Carter of a 2nd term. Ronald Reagan and George H. W. Bush were the front-runners. One of Reagan's proposals was an idea where tax cuts for the rich would "trickle down" to help the middle class and poor by "creating jobs". It was initially referred to as "supply-side economics". Bush referred to it as "voodoo economics", and it came to be known as "Reaganomics" - as well it should be.
Reagan won, and the Republicans have been beating this dead horse ever since.