The simplest of simple Keynesian economics on a napkin:
Let 0<b<1, where b is a constant, the marginal propensity to consume. Specifically, let b = 0.9
The government spending multiplier is 1/(1-b), therefore = 10
The tax multiplier, -b/(1-b) is therefore = -9
For ease, break the current stimulus policy down to $450 billion in new government spending, and $300 billion in new tax cuts. Using our multipliers, and other things constant, then the stimulus package would, in the limit, generate an additional
$4.5 trillion + 2.7 trillion = $7.2 trillion dollars.
Stones into bread. Some $750 billion stimulus package can increase aggregate demand up to $7.2 trillion.
2008 nominal GDP was $14.3 trillion. The administration's Keynesian economists are acting as if, without the stimulus, 2009 nominal GDP would fall 50%.
But, with the intervention, our economy should be on track by the end of the year.
So, Congressmen and women, go forth and multiply.