As the Post editorial board points out, Paul Ryan’s speech at the Republican National Convention Wednesday night was full of distortions. But which were the most dangerous?

It’s one thing to be misleading in campaign rhetoric that you put away and forget after election day. It’s another to be misleading in a way that perilously warps the country’s policy debate, a domain in which Americans’ understanding of the facts and their politicians’ willingness to compromise have real-world effects on programs and people. Ryan’s typical, fingers-in-ears approach to taxes was one example of the latter. And then there was this line:

“The greatest threat to Medicare is Obamacare.”

(Charles Dharapak/AP)

But Ryan’s ploy deeply betrays the substance, and the substance really matters. Medicare — not the hated stimulus, not TARP, not Solyndra — is the central hazard to America’s finances. It’s so big that Ryan’s claim that cutting a mere $716 billion out of Medicare could be some sort of existential danger to the program is laughable. In fact, the greatest threats to Medicare are unsustainable health-care cost growth and rising enrollments, outstripping the federal government’s ability to pay, not Obamacare’s mild attempt to scale back the spending. At other times and in other ways, Ryan has made this case.

It’s not just that Ryan’s Wednesday-night nonsense about Obamacare was flatly wrong — it’s that he fostered the very Mediscare demagoguery he is supposedly dedicated to fighting, the very illogic that has kept Washington from much of anything to Medicare except make it more expensive. Ryan claimed Wednesday that he would not duck hard issues, but he made honestly facing hard issues only more politically impossible. He told those Democrats who have flirted with anything like Medicare reform to stay away, because they can’t trust even Paul Ryan, who should know better, not to use it against them later. And he encouraged voters to believe that reform doesn’t require, well, reform. Ryan did not just misinform — he was corrosive.