At a Friday press conference, President Obama repeated many of the good things he said about the debt earlier this week : America's leaders should aim to strike a grand bargain on long-term federal debt; study after study after study has suggested the sorts of reforms necessary to get there, which include spending cuts and revenue increases; cut too little spending and America ultimately might not be able to afford to make forward-looking, cost-effective investments in things such as medical research; cut too much and those very investments will have to be the lines excised from the budget; for the math to work out, changes to Medicare and Social Security — including some Democrats hate — must be on the table; and, critically, both party’s leaders have to be willing to upset their respective bases to get any of this.

“We know how to get there,” the president said. And he’s right: Anyone who has ever played with the New York Times's brilliant budget balancing tool knows how easy it should be for reasonable people of good will to rationalize America's finances. Who, he was clearly saying, is the unreasonable party in these negotiations?

Yet the president also got one critical bit wrong: Why we have to do any of this in the first place.

(Evan Vucci/AP)

But a central, long-term driver of federal deficits isn't Iraq, and it's not the stimulus, either. It's demography combined with health-care cost inflation. The number of Americans eligible for Medicare and Social Security is set to double over the next 20 years. At the same time, health-care costs are expanding rapidly, a particular problem for the fee-for-service Medicare system. Unchecked, mandatory federal spending on Medicare, Medicaid and the State Children's Health Insurance Program will eat up 9 percent of Gross Domestic Product by 2030, nearly double its share today. Nine percent of the economy. If only the problem were merely cleaning up after George W. Bush.

That's why Obama's line, though politically attractive, is self-defeating if he is really serious about curbing entitlement costs, as he now claims to be — and as he should be. It encourages Democrats to wonder why they should touch the entitlements they love when they can just rescind the policies of a president they hated. It discourages Republicans from negotiating in good faith, since it sounds like Obama is making a partisan point. And it doesn't help Americans struggling to understand the problem, making it seem as though rolling back Bush-era policy will be enough to fix America's fiscal future, when such a massive part of the long-term budget problem is the health-care spending that Congress built into the federal budget years before Bush even thought about running for president.