Republicans and Democrats are being equally nasty in their campaign rhetoric, but they’re not being equally truthful. To cite one example, much of what the GOP is saying about Medicare simply isn’t supported by the facts.

Former New Hampshire Gov. John Sununu, one of Mitt Romney’s more pugnacious surrogates, almost had a conniption fit Tuesday when CNN’s Soledad O’Brien pressed him on his assertion that President Obama “gutted Medicare by taking $717 billion out of it.” As Sununu knows but refused to acknowledge, this is not true.

The claim is part of an attempt to shore up a vulnerability Romney created by choosing Paul Ryan as his running mate. The budget that Ryan authored, and convinced House Republicans to pass, would eventually change Medicare into a voucher program: Seniors would be given a certain amount of money each year to buy health insurance.

If that amount isn’t enough to pay for the kind of coverage you want or need — under Ryan’s latest plan, you could buy a policy from a private insurer or buy Medicare from the government — you pay the difference out of pocket. According to the Congressional Budget Office, the average Medicare recipient would pay an extra $6,500 a year.

It doesn’t take a genius to recognize the political problem this causes for Romney, especially in states where older voters are a key voting bloc. The Romney campaign decided to deal with the anticipated Democratic onslaught by striking first with the claim that it is Obama, not Romney, who wants to take away your Medicare.

Like many lies, this one uses a grain of truth as raw material. The Affordable Care Act, otherwise known as Obamacare, slows the rate of growth of payments to Medicare service providers by more than $700 billion over a decade. But no impact is felt by seniors themselves, whose benefits and costs remain the same.

When O’Brien reminded Sununu of these facts, he barked that she should “put an Obama bumper sticker on your forehead.” But the claim that Obama had “gutted Medicare” remained false, and O’Brien told Sununu: “You can’t just repeat it and make it true, sir.”

Sununu knows that. He just wants the focus to be on Romney’s pledge to undo Medicare “cuts” that aren’t really cuts at all. The Romney campaign knows that as long as people are trying to sort out the facts of this specific allegation, they aren’t talking about the real differences between the two parties on the very nature of entitlement programs such as Medicare.

The fact is that Ryan’s plan — which Romney has semi-endorsed and now cannot credibly disown — effectively would “end Medicare as we know it” — the phrase voters will hear a bazillion times between now and Election Day, especially those who live in Florida. This is one piece of campaign rhetoric that happens to be true.

Medicare as we know it is a form of social insurance, a guarantee that citizens over 65 will have adequate medical care regardless of how healthy they are or how much money they have. Medicare, as the GOP wants it to be, is a voucher program that will be adequate for some seniors and inadequate for others.

Ryan prefers the term “premium support” to describe his plan. But as Gertrude Stein surely would have noted, a voucher is a voucher is a voucher.

Everyone agrees that something has to be done about skyrocketing costs for Medicare and also Medicaid, the health care program for the poor. Obama took the first step toward “bending the curve” of cost increases with the Affordable Care Act. Leave aside, for the moment, the fact that Romney now pledges to undo the progress Obama has made. The question is what do we want Medicare to be?

There is no reason why Medicare cannot be reformed as a social insurance program. Other industrialized countries provide universal health coverage for their entire populations for a fraction of what we spend in the United States, and those other countries achieve equal or better health outcomes. Surely we can continue to do so for those of retirement age — if we still want to.

The question to ask Romney is whether he believes in social insurance — whether his objections to the way Obama has begun to reform Medicare are fiscal or ideological. Ask him and Ryan whether they agree that markets are often efficient but seldom compassionate. Ask him whether he sees the free market as our servant or our master.

Eugene Robinson’s e-mail address is eugenerobinson(at)washpost.com.