Because rate cuts are about the only "weapon" Democrats have to really boost the economy? And Republicans are "wasting" ammunition of their weapon, so they criticize?
Rate cuts tend to occur during a weak economy. If the Dems take over, they'll constantly say "look what we inherited from the GOP that has to be cleaned up". If Bloomberg wins as an independent, he'll say the same thing.
Dem's are desperately hoping the economy bombs between now and next November so they can run on the terrible republican economy. It really doesn't matter what the economy does, since they will just lie about it being the "worst economy in 40 years" or some such tall tale. Most voters are too dumb to know the difference.