Is there theoretical justification for the stimulus?

The idea of economic stimulus through large spending is mostly just a theoretical proposition, with few actual historical examples to look to. Probably the only examples that one can turn to are the huge spending increase of Hoover, the New Deal of Roosevelt, the US entrance into the second World War, and Japan's stimulus spending in the 90s. The only one that seems to confirm the Keynesian paradigm is the example of the US in World War II (and even this criticized, for example by Robert Higgs, who questions whether the US "wartime prosperity" was real prosperity or is simply an illusion created by over reliance on economic indicators which at the time were unreliable). All the examples of when stimulus didn't work are usually criticized because not enough money was spent or because other ill-advised policies were intermixed. For example, last week the NY Times had an article about how some economists think that the attempted stimulus in Japan in the 90s was a failure because the stimulus wasn't big enough. The article maintains some critical distance, since this is very controversial in the economic community.

What does this mean for our current situation though? The fear is that we'll have the experience of Japan in the 90s, with trillions wasted and nothing to show for it. Even if we assume that their stimulus was merely too small, is the current stimulus big enough? Keynesian defenders of the current bill have also been criticizing it for being too small. According to theory, the current stimulus should actually be enlarged considerably for it to work.

Even if this was politically feasible, though, this is not practicable. Where do you get all that money? Obviously, we have to borrow it, but from whom? The current economic downturn is not local, and the rest of the world does not have a that much surplus income to lend to us. There is a question of whether we will be able to even borrow enough to cover the current trillion dollar budget deficit plus the 800 billion dollar stimulus package. You try to increase the stimulus to an amount that is supposed to be large enough to stimulate the economy and you guarantee that we won't find enough borrowers to cover our costs.

So, what theoretical justification of the economic stimulus is there left? The Keynesians must concede that it is too small and we can't borrow enough to make it bigger. And the non-Keynsians prefer different measures, and certainly oppose the voluminous pork in the bill. Does anyone with any expertise on this subject actually support this highly risky, blind spending spree?