For one thing, repeal of Obamacare would cause older Americans to lose a number of significant benefits that the law provides, including the way it closes the “doughnut hole” in drug coverage and the way it protects early retirees.

Beyond that, the promise of unchanged benefits for Americans of a certain age just isn’t credible. Think about the political dynamics that would arise once someone born in 1956 still received full Medicare while someone born in 1959 couldn’t afford decent coverage. Do you really think that would be a stable situation? For sure, it would unleash political warfare between the cohorts — and the odds are high that older cohorts would soon find their alleged guarantees snatched away.

The question now is whether voters will understand what’s really going on (which depends to a large extent on whether the news media do their jobs). Mr. Ryan and his party are betting that they can bluster their way through this, pretending that they are the real defenders of Medicare even as they work to kill it. Will they get away with it?