AN UNPERSUASIVE DEFENSE OF THE INDEFENSIBLE…. Not surprisingly, there haven’t been too many kind words from the left for Paul Ryan’s House Republican budget plan this week. When a right-wing lawmaker propose eliminating of Medicare, gutting Medicaid, slashing taxes on the very wealthy, and imposing excessive hardship on the middle class, that’s generally a predictable outcome.

But Slate‘s Jacob Weisberg takes the unexpected, counter-intuitive path, suggesting that liberals “might want to consider whether some of what [Ryan] proposes doesn’t in fact serve their own ultimate goals.”

Ryan’s proposal to turn Medicare into a voucher provides an easy political target. But it’s hard to make a principled liberal case for the program in its current form. To do so, you have to argue that government-paid health care should be a right only for people over the age of 65, and for no one else. Medicare covers doctor and hospital bills at 100 percent, regardless of income. This gives doctors and patients an incentive to maximize their use of the system and waste public resources. Choosing to pay 100 percent of Warren Buffett’s medical bills while cutting Head Start reflects a strange set of social priorities, to say the least.

I suppose, but that’s not what the left is thinking. Progressives don’t want to cut Head Start while protecting Medicare — they want to protect both, and accurately believe neither should be subjected to unnecessary assaults from congressional Republicans.

As for the left making a principled case that “government-paid health care should be a right only for people over the age of 65, and for no one else,” that’s not even close to what the liberal mainstream believes. For one thing, a significant chunk of the left wants Medicare for all. For another, another significant chunk of the left fought to create the Affordable Care Act, which the last time I checked, brought government-paid health care to more than 30 million uninsured Americans under the age of 65.

Weisberg also argues that Ryan’s Medicare privatization scheme wouldn’t be that bad, since the vouchers that would go to seniors would be pretty generous — roughly the same as the “government now spends on Medicare.” That’s true, but it misses the point. As has been widely discussed this week, the costs of seniors’ care would continue to go up, but the value of the voucher would not. It’s one of the key, underlying points of the larger criticism.

Weisberg goes on to explain that Medicare needs some reforms to ensure its long-term fiscal health, which is certainly fair, but fails to explain why Ryan’s proposal to eliminate Medicare and replace it with a privatized voucher system is the appropriate “reform.”

As Jon Chait explained, Weisberg’s entire defense is “misguided.”

Privatizing Medicare is expensive. You add a layer of cost with no corresponding savings. You also introduce all the adverse selection problems that make insuring the elderly so difficult, and which required the establishment of Medicare in the first place. So the liberal case for keeping down Medicare costs ought to rest on beefing up the cost controls in the Affordable Care Act — Ryan, by the way, would eliminate those, too.

I can appreciate counter-intuitive arguments as much as the next guy, but Weisberg’s pitch falls flat.

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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.