So the proposal is exactly as President Obama described it: a proposal to deny health care (and many other essentials) to millions of Americans, while lavishing tax cuts on corporations and the wealthy — all while failing to reduce the budget deficit, unless you believe in Mr. Ryan’s secret revenue sauce. So why are centrists rising to Mr. Ryan’s defense?

Well, ask yourself the following: What does it mean to be a centrist, anyway?

It could mean supporting politicians who actually are relatively nonideological, who are willing, for example, to seek Democratic support for health reforms originally devised by Republicans, to support deficit-reduction plans that rely on both spending cuts and revenue increases. And by that standard, centrists should be lavishing praise on the leading politician who best fits that description — a fellow named Barack Obama.

But the “centrists” who weigh in on policy debates are playing a different game. Their self-image, and to a large extent their professional selling point, depends on posing as high-minded types standing between the partisan extremes, bringing together reasonable people from both parties — even if these reasonable people don’t actually exist. And this leaves them unable either to admit how moderate Mr. Obama is or to acknowledge the more or less universal extremism of his opponents on the right.

Enter Mr. Ryan, an ordinary G.O.P. extremist, but a mild-mannered one. The “centrists” needed to pretend that there are reasonable Republicans, so they nominated him for the role, crediting him with virtues he has never shown any sign of possessing. Indeed, back in 2010 Mr. Ryan, who has never once produced a credible deficit-reduction plan, received an award for fiscal responsibility from a committee representing several prominent centrist organizations.

So you can see the problem these commentators face. To admit that the president’s critique is right would be to admit that they were snookered by Mr. Ryan, who is the same as he ever was. More than that, it would call into question their whole centrist shtick — for the moral of my story is that Mr. Ryan isn’t the only emperor who turns out, on closer examination, to be naked.

Hence the howls of outrage, and the attacks on the president for being “partisan.” For that is what people in Washington say when they want to shout down someone who is telling the truth.