I could point out that the largest expansion of the US middle class on record happened during the 1950s when the top personal income tax rate was over 90%. But that could just be coincidence as well.
I can point out that economic theory says that the economic impact of giving a person a dollar is directly connected to their likelihood of spending it. Since the rich are less likely to spend than the middle class and the poor, taxing them hurts the economy less than taxing anyone else. But once you descend into economic theory, everyone has a theory and an argument and you get a morass that normal people can't make heads or tails of.
I could point out that the Republican ideology that less taxes on the rich is always good has exactly no evidence supporting it. That's a very thin amount of evidence for the huge social experiment that we're currently running in this country. But once you descend into politics everyone already has their minds made up, so that's a lost cause.
All of which begs the real question; if the proposition is true, what evidence would suffice to actually convince people of it who are not already convinced?