With the Republican presidential candidates headed to retiree-heavy Florida in advance of its January 31 primary, expect the debate about Medicare, the federal health care program for the elderly, to heat up.

If voters start focusing on the issue, it’s not going to be particularly helpful for Mitt Romney, the former governor of Massachusetts.

Mr. Romney raised the issue twice the other night in a debate on CNN. First, he said, “we do not want a $500 billion cut in Medicare to pay for ‘Obamacare.’” Then he called ObamaCare a “Medicare-cutting monster.” In the same debate, he faulted Mr. Obama for “making us more and more like a European social-welfare state. … making us an entitlement society.”

At the time, I noted the apparent inconsistency. How can Mr. Romney simultaneously fault President Obama for cutting $500 billion from Medicare and for turning us into a European-style social-welfare state or entitlement society?

But inconsistency is only the beginning of the problems with Mr. Romney’s Medicare scare.

Inaccuracy is another problem. According to the federal Office of Management and Budget, Medicare spending in 2008, the last year of the George W. Bush administration, was $391 billion. In 2009, the first year of President Obama’s administration, it was $430 billion. In 2010, it was $452 billion (larger than the entire federal budget in 1977, in nominal dollars). For 2011, it is estimated to be $494 billion. In 2013, it is estimated at $534 billion.

So, just to be precise, far from cutting $500 billion from Medicare, President Obama has, in a mere three years, managed toincrease Medicare spending to $494 billion in 2011 from $391 billion in 2008. That is an increase of $103 billion, or 26% in three years.

A Republican candidate genuinely committed to free enterprise and balanced budgets, as Mr. Romney claims he is, and genuinely opposed to a European-style social welfare state or entitlement society, as Mr. Romney claims he is, would be faulting Mr. Obama for spending too much on Medicare, not for cutting it.

Sure, there may be some kind of insider Washington definition of an imaginary “cut” involving decade-long projections and baselines and assumed growth rates under which the targeted growth rate for Medicare spending under ObamaCare is less than what it was before ObamaCare. Such savings are more often assumed than achieved. But that’s the sort of definition of a budget cut that the American Federation of State, County, and Municipal Employees used in its Mediscare television ads against Newt Gingrich and House Republicans in the 1990s, not a definition you’d expect from a Republican presidential candidate or from someone concerned about government spending.

Beyond the accuracy and consistency problems, there’s a governing problem. If Mitt Romney actually ever wins the Republican nomination and then manages to get elected president, he’s going to have to try to follow through on his pledge to reduce the size of government and move toward a balanced budget. Medicare is the third-biggest federal government expense after defense and Social Security, and it’s going to be very difficult for any president to get a balanced budget, or even anywhere close to one, without reining in the growth of the Medicare program in a way that opponents will be able to demonize as “cuts.” The way to succeed at doing it would be to educate the electorate now about the problem of runaway Medicare costs, not to pander to them by criticizing President Obama for threatening their entitlements.

Nor was Mr. Romney’s remark on the matter in the South Carolina debate a casual one. In a December 9, 2011, interview with the Des Moines Register, Mr. Romney said, “the only person I know of to ever cut Medicare is President Obama, by $500 billion to fund Obamacare. … I don’t know anyone among Republicans who’s talking about cutting it. The only person who’s cut Medicare for current Medicare recipients is President Obama.”

At a Republican presidential forum on September 5, 2011, Mr. Romney, explaining the difference between RomneyCare and ObamaCare, said, “We didn't cut Medicare. One president in modern history cut Medicare, this president and I'll say to him, why don't you give me a call and I'll tell you what to do right and what not to do.”

It’s a regular campaign talking point for Mr. Romney.

None of this is to say that there aren’t legitimate concerns about what ObamaCare will do to, say, Medicare Advantage plans that many seniors like, or to reimbursement rates for certain doctors, hospitals, or procedures.

If Mr. Romney isn’t more careful, though, his attacks on Medicare “cuts” risk scaring voters away not from President Obama, but from Mr. Romney’s own campaign.

Ira Stoll is editor of FutureOfCapitalism.com and author of Samuel Adams: A Life.