The short of it is this: cutting taxes to promote economic growth never worked and cannot work. But that doesn't stop conservative ideologues; they just concoct more lies in an attempt to make the bad look good and the truly terrible look even better. As one of my favorite Sixties pop musicians put it, "Lie la lie; lie la lie lie, lie la lie, lie la lie..."
AFTERTHOUGHT: As Krugman hinted and TPM's Daniel Strauss spelled out, GOPers are preparing to wreck the CBO so as to use something they call "dynamic scoring," which basically means assuming/predicting economic growth as a direct consequence of lower taxes. No evidence in practice has ever been presented demonstrating this assertion/assumption, but hey, it fits with their ideology...