Put it this way: If a Republican president had managed to extract the kind of concessions on Medicare and Social Security that Mr. Obama is offering, it would have been considered a conservative triumph. But when those concessions come attached to minor increases in revenue, and more important, when they come from a Democratic president, the proposals become unacceptable plans to tax the life out of the U.S. economy.

Beyond that, voodoo economics has taken over the G.O.P.

Supply-side voodoo — which claims that tax cuts pay for themselves and/or that any rise in taxes would lead to economic collapse — has been a powerful force within the G.O.P. ever since Ronald Reagan embraced the concept of the Laffer curve. But the voodoo used to be contained. Reagan himself enacted significant tax increases, offsetting to a considerable extent his initial cuts.

And even the administration of former President George W. Bush refrained from making extravagant claims about tax-cut magic, at least in part for fear that making such claims would raise questions about the administration’s seriousness.

Recently, however, all restraint has vanished — indeed, it has been driven out of the party. Last year Mitch McConnell, the Senate minority leader, asserted that the Bush tax cuts actually increased revenue — a claim completely at odds with the evidence — and also declared that this was “the view of virtually every Republican on that subject.” And it’s true: even Mr. Romney, widely regarded as the most sensible of the contenders for the 2012 presidential nomination, has endorsed the view that tax cuts can actually reduce the deficit.

Which brings me to the culpability of those who are only now facing up to the G.O.P.’s craziness.

Here’s the point: those within the G.O.P. who had misgivings about the embrace of tax-cut fanaticism might have made a stronger stand if there had been any indication that such fanaticism came with a price, if outsiders had been willing to condemn those who took irresponsible positions.

But there has been no such price. Mr. Bush squandered the surplus of the late Clinton years, yet prominent pundits pretend that the two parties share equal blame for our debt problems. Paul Ryan, the chairman of the House Budget Committee, proposed a supposed deficit-reduction plan that included huge tax cuts for corporations and the wealthy, then received an award for fiscal responsibility.

So there has been no pressure on the G.O.P. to show any kind of responsibility, or even rationality — and sure enough, it has gone off the deep end. If you’re surprised, that means that you were part of the problem.