Barack Obama blundered on Saturday in saying Americans “can’t eat as much as we want” any more. His adulatory crowds may never get it, but he disclosed something every other voter needs to know about him.

Senator Obama told a crowd in Oregon last week, “We can’t drive our SUVs and, you know, eat as much as we want and keep our homes on, you know, 72 degrees all the time, whether we’re living in the desert or we’re living in the tundra, and then just expect every other country is going to say OK.”

He probably didn’t think this exceptionable because a lot of people — the kind you’d meet over the arugula bin at the food co-op in Hyde Park — figure it needs to be said: It’s so embarrassing, what the Europeans must think of our Land of the Larded. All those NASCAR yahoos are eating too much, and someone ought to say it.

Or there are the nostalgic sorts who say that if this war on terror were real, we’d reinstitute food rationing, just like when we beat Hitler. Never mind the illogic of it in a land that routinely produces mountains of surplus crops. Your triglycerides, goes this strain of thought, indicate that America really could stand some federal dietary discipline.

Bad enough to hear sanctimony about diet from a thin candidate. But the scope of Mr. Obama’s error will grow as his meaning sinks in. Unlike World War II, this would be a permanent lessness unlimited by any VE Day. That he mentioned cars and furnaces suggests his impulse to regulate Americans’ appetites could include anything consumable. And under it all, Mr. Obama shows a disturbing notion of just who gets to say, “Enough.”

Take the idea that America’s eating too much. You may eat too much. My 11-year-old doesn’t. I don’t know where he’s packing all that spaghetti, but it’s not into width. I work with a really thin person: She can’t be eating too much. What about joggers? All that exercise builds an appetite. If the problem is worldwide scarcity, this is as wasteful as fueling the Escalade for a night of cruising.

A lot of people will get this the moment they think, rather than feel: The trouble with saying America eats too much is that we don’t have a collective mouth. We have 300 million individual ones, of varying degrees of sinful gluttony. Public policy is too blunt an instrument to redeem them.

The correct safeguard against such personal, individual failings is personal, individual morality, bounded by social expectation, not legal commands. This would be obvious had the left not spent the past two centuries emasculating any extragovernmental institution, especially religion. Religion, which invented the notion that gluttony is sin, is how free societies set moral boundaries without letting the law put an iron collar on every human decision.

But now, in policy we trust. So Mr. Obama thrills his Whole Foods base and spooks everyone else because they suspect he’s fixing to have some form of government decide what “too much” is. Or whether you need your SUV, or the children you haul in it. Or a sweater instead of a warm house, and it doesn’t help that Jimmy Carter already let us all know that Democrats feel perfectly OK with doing just this.

And we’d do this just so other countries feel nicer about us. Who said your grocery list is their business? To add some context, Mr. Obama said you can’t drive, eat, or heat as you wish if our government is to credibly harangue China into accepting slower growth. But why would we presume to, anyhow? If we’re disliked now for our Big Macs, imagine how loathed we’ll be when, having gotten thin, our chief export is self-righteous lectures for countries still trying not to starve.

Mr. Obama’s gaffe exposes the default pessimism of his base, emotionally drawn to constraint, whatever the excuse. This supposed food crisis, even as Japan destroys mountains of rice for protectionist purposes and America mandates that corn become ethanol, is the result of governments’ stupidity. Surely some governmental modesty is in order.

Here’s why Washington and Mr. Obama and his swooning rally-goers will never be able to say what “too much” on the dinner plate is. A while back, my wife and I took the kids on an adventure to New York. The Staten Island ferry was a bargain, but we assumed we’d get rooked on food, especially in Manhattan.

So, we figured, if we’re overpaying for dinner, why not do it for something we can’t get at home? We marched the kids into a kosher restaurant. And indeed, the prices looked high.

Then the food came. And came and came, dish after dish. The lady running the place gave us sherbet, unbidden. She could barely find a place to set it.

Strictly speaking, it was too much food. But, I’m guessing, a veteran restaurateur could spot out-of-towners who had slumping, late-afternoon kids and figured that, on top of big portions, a little dessert would be welcome. Maybe she was cultivating repeat business. Maybe she just wanted us to think of her restaurant and her town as fondly as she did. I prefer to think the latter.

For that purpose, such oversized generosity was just right. That’s an old logic understandable to any reasonable person yet opaque to any calorie-rationing regulation ever written.

Mr. McIlheran is a columnist for the Milwaukee Journal Sentinel.