A dominant theme of President Obama's budget speech last Wednesday was that our fiscal problems would vanish if only the wealthiest Americans were asked "to pay a little more." Since he's asking, imagine that instead of proposing to raise the top income tax rate well north of 40%, the President decided to go all the way to 100%.

Let's stipulate that this is a thought experiment, because Democrats don't need any more ideas. But it's still a useful experiment because it exposes the fiscal futility of raising rates on the top 2%, or even the top 5% or 10%, of taxpayers to close the deficit. The mathematical reality is that in the absence of entitlement reform on the Paul Ryan model, Washington will need to soak the middle class—because that's where the big money is.

Say we take it up to the top 10%, or everyone with income over $114,000, including joint filers. That's five times Mr. Obama's 2% promise. The IRS data are broken down at $100,000, yet taxing all income above that level throws up only $3.4 trillion. And remember, the top 10% already pay 69% of all total income taxes, while the top 5% pay more than all of the other 95%.