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.

Consider the Internal Revenue Service’s income tax statistics for 2008, the latest year for which data are available. The top 1% of taxpayers—those with salaries, dividends and capital gains roughly above about $380,000—paid 38% of taxes. But assume that tax policy confiscated all the taxable income of all the “millionaires and billionaires” Mr. Obama singled out. That yields merely about $938 billion, which is sand on the beach amid the $4 trillion White House budget, a $1.65 trillion deficit, and spending at 25% as a share of the economy, a post-World War II record.

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%

We recognize that 2008 was a bad year for the economy and thus for tax receipts, as payments by the rich fell along with their income. So let’s perform the same exercise in 2005, a boom year and among the best ever for federal revenue. (Ahem, 2005 comes after the Bush tax cuts that Mr. Obama holds responsible for all the world’s problems.)

In 2005 the top 5% earned over $145,000. If you took all the income of people over $200,000, it would yield about $1.89 trillion, enough revenue to cover the 2012 bill for Medicare, Medicaid, and Social Security—but not the same bill in 2016, as the costs of those entitlements are expected to grow rapidly. The rich, in short, aren’t nearly rich enough to finance Mr. Obama’s entitlement state ambitions—even before his healthcare plan kicks in.

So who else is there to tax? Well, in 2008, there was about $5.65 trillion in total taxable income from all individual taxpayers, and most of that came from middle income earners, and that is where Democrats are inevitably headed for the same reason that Willie Sutton robbed banks.

 —Wall Street Journal, Apr. 18, 2011, p. A14