Economists are just numerical politicians supporting political policy using empirical concepts unprovable and unproven and undecipherable to the common folks like you and me. When tax rates go up or down, the economy could very well be unaffected by it and so when their is growth, even with reduced rates, tax revenues go up and with declining growth (like now) tax revenue goes down).
"Economic proofs" offered by the left AND the right are merely used to support current political thought in order to convince the public to acquiese to proposed tax laws for one side or against the other.
Not that economics is totally useless, it keeps a great many academics from becoming politicians and that's a very good thing. www.joadcycle.com