If you want to know what a political party really stands for, follow the money. Pundits and the public are often deceived; remember when George W. Bush was a moderate, and Chris Christie a reasonable guy who could reach out to Democrats? Major donors, however, generally have a very good idea of what they are buying, so tracking their spending tells you a lot.

So what do contributions in the last election cycle say? The Democrats are, not too surprisingly, the party of Big Labor (or what’s left of it) and Big Law: unions and lawyers are the most pro-Democratic major interest groups. Republicans are the party of Big Energy and Big Food: they dominate contributions from extractive industries and agribusiness. And they are, in particular, the party of Big Pizza.

No, really. A recent Bloomberg report noted that major pizza companies have become intensely, aggressively partisan. Pizza Hut gives a remarkable 99 percent of its money to Republicans. Other industry players serve Democrats a somewhat larger slice of the pie (sorry, couldn’t help myself), but, over all, the politics of pizza these days resemble those of, say, coal or tobacco. And pizza partisanship tells you a lot about what is happening to American politics as a whole.

Why should pizza, of all things, be a divisive issue? The immediate answer is that it has been caught up in the nutrition wars. America’s body politic has gotten a lot heavier over the past half-century, and, while there is dispute about the causes, an unhealthy diet . fast food in particular . is surely a prime suspect. As Bloomberg notes, some parts of the food industry have responded to pressure from government agencies and food activists by trying to offer healthier options, but the pizza sector has chosen instead to take a stand for the right to add extra cheese.

The rhetoric of this fight is familiar. The pizza lobby portrays itself as the defender of personal choice and personal responsibility. It’s up to the consumer, so the argument goes, to decide what he or she wants to eat, and we don’t need a nanny state telling us what to do.

It’s an argument many people find persuasive, but it doesn’t hold up too well once you look at what’s actually at stake in the pizza disputes. Nobody is proposing a ban on pizza, or indeed any limitation on what informed adults should be allowed to eat. Instead, the fights involve things like labeling requirements . giving consumers the information to make informed choices . and the nutritional content of school lunches, that is, food decisions that aren’t made by responsible adults but are instead made on behalf of children.

Beyond that, anyone who has struggled with weight issues . which means, surely, the majority of American adults . knows that this is a domain where the easy rhetoric of “free to choose” rings hollow. Even if you know very well that you will soon regret that extra slice, it’s extremely hard to act on that knowledge. Nutrition, where increased choice can be a bad thing, because it all too often leads to bad choices despite the best of intentions, is one of those areas . like smoking . where there’s a lot to be said for a nanny state.

Oh, and diet isn’t purely a personal choice, either; obesity imposes large costs on the economy as a whole.

But you shouldn’t expect such arguments to gain much traction. For one thing, free-market fundamentalists don’t want to hear about qualifications to their doctrine. Also, with big corporations involved, the Upton Sinclair principle applies: It’s difficult to get a man to understand something when his salary depends on his not understanding it. And beyond all that, it turns out that nutritional partisanship taps into deeper cultural issues.

At one level, there is a clear correlation between lifestyles and partisan orientation: heavier states tend to vote Republican, and the G.O.P. lean is especially pronounced in what the Centers for Disease Control and Prevention call the “diabetes belt” of counties, mostly in the South, that suffer most from that particular health problem. Not coincidentally, officials from that region have led the pushback against efforts to make school lunches healthier.

At a still deeper level, health experts may say that we need to change how we eat, pointing to scientific evidence, but the Republican base doesn’t much like experts, science, or evidence. Debates about nutrition policy bring out a kind of venomous anger . much of it now directed at Michelle Obama, who has been championing school lunch reforms . that is all too familiar if you’ve been following the debate over climate change.

Pizza partisanship, then, sounds like a joke, but it isn’t. It is, instead, a case study in the toxic mix of big money, blind ideology, and popular prejudices that is making America ever less governable.