Keeping it simple:
The strongest argument for the stimulus (and the one that I think, in their heart-of-hearts, most supporters actually hold) is what could be called the Costanza-Hoover Principle: do the opposite of whatever Herbert Hoover did. In a world of limited knowledge, this isn’t as crazy as it might seem, at least as a starting point. It sure seems like Hoover screwed up; and hopefully we can avoid his mistakes. This pretty much boils down to: avoid a tariff war; don’t try to balance the budget right now; don’t restrict the money supply (that gold standard thing is right out); and, most importantly, prevent a collapse of the banking system.