Economist Greg Mankiw, a former Bush administration official and current advisor to Mitt Romney’s campaign, argued over the weekend that the two major parties share a “few fundamental agreements” on health care policy, and have even “stumbled into consensus.”

How is this possible? Take Mankiw’s first, and arguably worst, point.

Representative Paul D. Ryan, Republican of Wisconsin, has attracted much attention with his plan to reform Medicare. He proposes replacing the current fee-for-service program, in which the government picks up the bill for medical expenses, with a “premium-support” system in which seniors use federal dollars to choose among competing private insurance plans.

Democratic critics of the plan suggest that enacting it would be akin to pushing Grandma over a cliff. But they rarely point out that the premium-support model is in some ways similar to the system set up under President Obama’s health care law. If choosing among competing private plans on a government-regulated exchange is a good idea for someone at age 50, why is it so horrific for someone who is 70?

Mark Kleiman has a good piece on this — Mark asks if Mankiw is “really as stupid as he pretends to be” — but I wanted to walk through the ways in which Mankiw’s argument is wrong, because I think this is important.

Right off the bat, his piece characterizes the Ryan plan as “premium support,” which really isn’t accurate. More important, though, is the fact that Mankiw is drawing parallels between the Affordable Care Act and the House Republican plan for Medicare privatization.

These parallels aren’t wrong. The problem, though, is with the premise: if Dems like the ACA for everyone below Medicare eligibility, why shouldn’t Dems like a very similar model for seniors?

Mankiw isn’t a dumb guy, which is why it’s odd to see him put this in a New York Times op-ed. If he’s followed the debate at all, he has to know the answer to his own question. For Democrats, bringing people into the health care system with affordable coverage is the key. Most Dems would love to do Medicare for All, but that doesn’t appear to be an option right now. Left with two choices — leave millions of Americans with nothing or subsidize their heavily-regulated private insurance — Democrats gladly prefer the latter.

But Medicare is a system of socialized medicine that works and is wildly popular. Democrats created Medicare; Democrats have protected Medicare; Democrats have championed Medicare; and Democrats have made Medicare part of the very fabric of the party. Paul Ryan and nearly every Republican in Congress want to scrap the program altogether and replace it with something worse — forcing seniors into the private system with subsidies that won’t keep up with escalating costs.

Yes, Ryan’s model for seniors would look similar to an ACA-style system. But it would still be a step backwards, whereas the ACA is a step forward for everyone else.

Mankiw is asking the wrong question. The more salient point is, why would Dems want to scrap a Medicare program that costs less than the private system and already works as it should?

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Follow Steve on Twitter @stevebenen. Steve Benen is a producer at MSNBC's The Rachel Maddow Show. He was the principal contributor to the Washington Monthly's Political Animal blog from August 2008 until January 2012.