By PAUL KRUGMAN

Paul Ryan’s speech Wednesday night may have accomplished one good thing: It finally may have dispelled the myth that he is a Serious, Honest Conservative. Indeed, Mr. Ryan’s brazen dishonesty left even his critics breathless.

Some of his fibs were trivial but telling, like his suggestion that President Obama is responsible for a closed auto plant in his hometown, even though the plant closed before Mr. Obama took office. Others were infuriating, like his sanctimonious declaration that “the truest measure of any society is how it treats those who cannot defend or care for themselves.” This from a man proposing savage cuts in Medicaid, which would cause tens of millions of vulnerable Americans to lose health coverage.

And Mr. Ryan — who has proposed $4.3 trillion in tax cuts over the next decade, versus only about $1.7 trillion in specific spending cuts — is still posing as a deficit hawk.

But Mr. Ryan’s big lie — and, yes, it deserves that designation — was his claim that “a Romney-Ryan administration will protect and strengthen Medicare.” Actually, it would kill the program.

Before I get there, let me just mention that Mr. Ryan has now gone all-in on the party line that the president’s plan to trim Medicare expenses by around $700 billion over the next decade — savings achieved by paying less to insurance companies and hospitals, not by reducing benefits — is a terrible, terrible thing. Yet, just a few days ago, Mr. Ryan was still touting his own budget plan, which included those very same savings.

But back to the big lie. The Republican Party is now firmly committed to replacing Medicare with what we might call Vouchercare. The government would no longer pay your major medical bills; instead, it would give you a voucher that could be applied to the purchase of private insurance. And, if the voucher proved insufficient to buy decent coverage, hey, that would be your problem.

Moreover, the vouchers almost certainly would be inadequate; their value would be set by a formula taking no account of likely increases in health care costs.

Why would anyone think that this was a good idea? The G.O.P. platform says that it “will empower millions of seniors to control their personal health care decisions.” Indeed. Because those of us too young for Medicare just feel so personally empowered, you know, when dealing with insurance companies.

Still, wouldn’t private insurers reduce costs through the magic of the marketplace? No. All, and I mean all, the evidence says that public systems like Medicare and Medicaid, which have less bureaucracy than private insurers (if you can’t believe this, you’ve never had to deal with an insurance company) and greater bargaining power, are better than the private sector at controlling costs.

I know this flies in the face of free-market dogma, but it’s just a fact. You can see this fact in the history of Medicare Advantage, which is run through private insurers and has consistently had higher costs than traditional Medicare. You can see it from comparisons between Medicaid and private insurance: Medicaid costs much less. And you can see it in international comparisons: The United States has the most privatized health system in the advanced world and, by far, the highest health costs.

So Vouchercare would mean higher costs and lower benefits for seniors. Over time, the Republican plan wouldn’t just end Medicare as we know it, it would kill the thing Medicare is supposed to provide: universal access to essential care. Seniors who couldn’t afford to top up their vouchers with a lot of additional money would just be out of luck.

Still, the G.O.P. promises to maintain Medicare as we know it for those currently over 55. Should everyone born before 1957 feel safe? Again, no.

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?