Why is American politics so dysfunctional? Whatever the deeper roots of our distress, the proximate cause is ideological extremism: Powerful factions are committed to false views of the world, regardless of the evidence.

Notice that I said factions, plural. There’s no question that the most disruptive, dangerous extremists are on the right. But there’s another faction whose obsessions and refusal to face reality have also done a great deal of harm.

But I’m not talking about the left. Radical leftists are virtually nonexistent in American politics; can you think of any prominent figure who wants us to move to the left of, say, Denmark? No, I’m talking about fanatical centrists.

Over the past few days we’ve been treated to the ludicrous yet potentially destructive spectacle of Howard Schultz, the Starbucks billionaire, insisting that he’s the president we need despite his demonstrable policy ignorance. Schultz obviously thinks he knows a lot of things that just aren’t so. Yet his delusions of knowledge aren’t that special. For the most part, they follow conventional centrist doctrine.

Now, single-payer health care (actually called Medicare!) hasn’t bankrupted Canada. In fact, every advanced country besides America has some form of universal health coverage, and manages to afford it.

The real issue with “Medicare for all” isn’t costs — the taxes needed to pay for it would almost surely be less than what Americans now pay in insurance premiums. The problem instead would be political: It would be tricky persuading people to trade private insurance for a public program. That’s a real concern for Medicare-for-all advocates, but it’s not at all what either Schultz or Bloomberg is saying.

Finally, the hallmark of fanatical centrism is the determination to see America’s left and right as equally extreme, no matter what they actually propose.

Thus, throughout the Obama years, centrists called for political leaders who would address their debt concerns with an approach that combined spending cuts with revenue increases, offer a market-based health care plan and invest in infrastructure, somehow never managing to acknowledge that there was one major figure proposing exactly that — President Barack Obama.

And now, with Democrats taking a turn that is more progressive but hardly radical, centrist rhetoric has become downright hysterical. Medicare and Medicaid already cover more than a third of U.S. residents and pay more bills than private insurance.

But Medicare for all, says Schultz, is “not American.” Elizabeth Warren has proposed taxes on the wealthy that are squarely in the tradition of Teddy Roosevelt; Bloomberg says that they would turn us into Venezuela.

Where does the fanaticism of the centrists come from? Much of the explanation, I think, is sheer vanity.