The Constitution is a boardwalk over the mud of politics. Any step off of it, and you become mired in the most dangerous toil, always struggling for footing, always in danger of being sucked under.

The usual metaphor for this is “a slippery slope,” but you don’t need an incline to fall prey to the internecine in the modern “churning state.” All you need is mud, the usual muck from the unconstrained push and shove of politics.

One step in, and the likelihood of getting out is much less. Often, the best you can do is lose your boot. You always come out smelling worse than when you entered.

To what events do we owe this extended metaphor?

Michael Bloomberg’s latest crusade: his campaign against our alleged over-use of salt. He says (using old and contested evidence) that the nation is using way too much salt, to our serious detriment. That’s why he’s pushing New York City government to take the crusade against salt nationwide.

I kid you not.

In another venue — my Common Sense radio program and email letter — I mulled over Bloomberg’s audacity:

This would have been a strawman example — a reductio ad absurdum — a generation ago. Back then, when some of us objected to, say, regulation of cigarettes, arguing that next government would be regulating the salt on our French Fries, earnest nanny-state proponents would sniff. No. They wouldn’t do anything that absurd.

I had countless arguments like this when I was young. I used the example of salt on French fries — and similar “absurdities” — often. And I was just as often rebuked.

But of course, my “strawman” arguments were dead-on accurate, and my opponents in political argument were not merely wrong, but spectacularly wrong. I don’t know if those earnest nannies meant what they said at the time and merely didn’t know the extent to which they’d be sucked in by success and precedent to regulate, still further, the minutest of actions impinging on health, or whether they were lying through their teeth.

Whatever the reason, here we are now, decades after nannyism became a federal philosophy, and not only do we have vast bureaucracies guarding us from our own often very dear vices, but we have the mayor of New York working to put the screws on restaurants and food providers across the country.

For “our own good,” of course.

The truth is simple: Using forceful means to override our peaceful activities — no matter how personally harmful — is a dangerous precedent. The principle of liberty is fairly clear. Governments’ limited powers, as specified in the U.S. Constitution, were once clear, too.

And it looks to get a lot worse. Mark my “strawman” words, as soon as the federal government offers universal health care, the attempts to regulate the minutia of our activities for health reasons will dramatically increase. The reason is simple. When the government — using our money collected from taxes — claims to be paying for our care, every vice indulged or risk taken that might affect federal money spent will seem like a form of treason. And traitors must be hounded and rounded up.

It all stands to reason.

That is, if you have abandoned freedom and replaced it with the muck of politics.

The best we can expect of such a complete descent into the muck is to be healthy slaves.

But even that best take is an illusion. The bulk of the “safety” and “health” legislation now existing acts as a drag on actual well-being, even with freedom left out of the accounting.

The FDA’s “benevolent” hand on the development of drugs, for example, almost certainly kills more patients — by making them wait, as if they were already inhabitants of a country stuck in the mire of socialized medicine — than it helps by making drugs “safe.”

There’s a ton of literature on this, from economists who know what they are talking about. But you wouldn’t know it by political discourse. The FDA is healthy, robust, the better to make us all worse off.

And now, worming its way through Congress, is HR 875, a bill to further regulate and bureaucratize the food provision industry. It would establish a new bureaucracy, the Food Safety Administration, and it would require all sorts of paperwork ideally suited to raising the cost of food and further upping economies of scale, crowding out small farms and farmers’ markets.

And even church bake sales.

Since it could kill the locavore movement, it has a chance of failing. (A lot of people really want to eat outside the corporate-government hegemony.) But, if we lived under an honored Constitution, in this case a reasonable reading of the Commerce Clause, it would lack even the smallest chance of becoming law. The vibrancy of our small-scale and home enterprises would not be at risk at all, had not America long ago stepped off the boardwalk and into the muck of modern politics.

For our health as well as our prosperity and freedom, we need to raise the level of politics in this country — out of the muck.