Does the Constitution prohibit the Census from asking how many citizens America has? If that seems like the most cockeyed question you’ve heard, you might have missed the morning papers.

As many as 20 states — the big Democratic ones, in the main — are hatching a vast new legal campaign against the Trump administration. Ignorance is bliss, they say. They want to stop the administration from launching in 2020 a Census that would ask, among other things, whether one is a citizen. It might scare off undocumented immigrants from answering the Census.

“Fouling” the Census is the way the Washington Post described the administration’s plan. The New York Times trundled in the next day with an editorial saying the White House is “trifling” with the Census.

What they fear is that asking about citizenship could reduce the population tally of blue states and Democratic congressional districts. It could mean they’d lose out on government money — and representatives in Congress.

That fear looms in a letter from New York’s Attorney General Eric Schneiderman, and officers of 19 states. It was dispatched last month to Commerce Secretary Wilbur Ross, who oversees the Census.

The letter is twice as long as the Book of Lamentations. It alleges that adding a citizenship question “would significantly depress participation,” slighting states and cities “with large immigrant communities.” It warns “tremendous harms” would result.

Low participation in the Census, Schneiderman & Co. say, would “deprive our states of their fair share of hundreds of billions of dollars in federal funds that are allocated in part on decennial Census data.”

This would follow from the prospect that a depressed Census could “threaten our states’ fair representation in Congress” and “dilute our states’ role in the Electoral College.”

If that were true, though, one would think that New York’s representation in Congress would have improved when the federal Census takers stopped asking about citizenship — in 1950, the last year the citizenship question was in the constitutionally mandated 10-year Census.

At the time, the New York delegation in the House had 45 members. Today, it has 27. That means the state’s representation in the Electoral College has also collapsed to little more than half of what it was when the Census was asking about citizenship.

It’s not my intent here to suggest that dropping the citizenship question caused the collapse in the state’s representation in Congress and the Electoral College. Just that it didn’t help much, if any.

The winners in population growth — Texas, say, North Dakota, Utah and Florida — are low-tax, high-economic freedom states. That’s a context in which griping about the Census looks like a loser’s game.

Which brings me back to the Constitution. It’s true it doesn’t require the Census to ask anyone about citizenship. The Census it commands had to do with taxes and doling out seats in the House of Representatives. It was a count not of citizens but of “persons.”

“Representatives and direct Taxes shall be apportioned among the several States which may be included within this Union, according to their respective Numbers,” the Constitution’s actual original language says.

The numbers, it went on, “shall be determined by adding to the whole Number of free Persons, including those bound to Service for a Term of Years, and excluding Indians not taxed, three fifths of all other Persons.”

Meaning slaves.

That’s the notorious three-fifths clause, undone by the 14th Amendment. It stands today as a reminder that it was not citizens but persons that the Constitution counts for apportionment.

Neither, though, does the Constitution prohibit the Census from asking persons it’s counting whether they are citizens. There are reasons in the Constitution and in law where we might want a citizen count.

The 14th Amendment requires a state’s apportionment to be reduced if it denies the vote to certain citizens. Federal law limits federal voting to citizens. Both personhood and citizenship can be important.

I’m no lawyer, but I suspect that if the Supreme Court got ahold of this, the leftists would lose. Luckily for them, the final word on the Census rests with Congress, which they could yet control.

This column was first issued in the New York Post.