In the New Republic, the Brookings Institution’s William Galston zeroes in on the fuzzy math. “Unless Obama is prepared to tolerate huge deficits indefinitely,” he writes, “or to emulate arch-conservatives and curb the budget deficit with spending cuts only, he will have to break his unsustainable tax pledge at some point. The only question is when.”

More remarkable still, Mr. Galston was jumping off from an article in National Review by Reihan Salam, who made the same point about the mathematical impossibilities of Mr. Obama’s present tax pledge. Mr. Salam, a policy adviser at the pro-market think tank Economics 21, observes that the revenues Mr. Obama needs to pay for his agenda fall in the rung just below the super-rich—that is, Americans earning between $100,000 and $200,000. The political problem is that this is a block that went Republican by 56% to 43% in 2010…

This is the heart of the argument shaping up between Mr. Obama and Paul Ryan, chairman of the House Budget Committee. Manifestly Mr. Obama believes that as much as Americans say they want smaller government, the moment they find one of their favorite programs (e.g., Medicare) up for consideration, they balk.

Mr. Ryan and Republicans make the opposite bet: The president’s spending has made Americans more willing to face up to these choices, especially if the alternative is higher taxes on more people.