The people who oppose putting a citizenship question on the 2020 decennial census have no idea what a potential mess they are creating.

The controversy over the question got a lot more complicated and interesting last week, when President Trump did an end-around and said he’d use other citizenship data collected by the Census Bureau in place of the decennial question.

I might take some credit for that because I wrote, in a column on June 18, that this other data existed from employment surveys and something called the American Community Survey, which is a sister poll to the decennial that is conducted constantly.

Since I know Trump reads The Post, that might be what gave him the idea for his maneuver around a recent Supreme Court ruling that — at least temporarily — prohibited asking people if they are citizens on next year’s constitutionally mandated census.

How could this mess get any worse? Easily, because the government asks about citizenship in its applications for food stamps and other benefits that come from the US Department of Health and Human Services (HHS).

The question of a person’s citizenship has been on and off the 10-year census since the beginning. The question was pulled in 1950 — but only, it seems, to make the survey more compact.

The Trump administration wants the question back in the census in 2020. But the Democrats (and I suppose others) argue that asking about whether people are US citizens would make some immigrant groups reluctant to answer questions.

Politically speaking, that might undercount the groups of people who support the Democrats and could lead to states like California losing representation in the House.

The Republicans, for their part, hope that’s exactly what happens.

The Supreme Court recently said the citizenship question couldn’t be on the census after the Trump administration failed to show it followed the proper procedures. And the decision came down too late for the Census Bureau to start the process anew before the deadline for forms to be printed.

Just so you know (if you care), I think the citizenship question is pertinent to any census. In fact, it might be the only essential question that needs to be asked.

Here’s the new wrinkle.

Each and every state must ask the citizenship question when people apply for benefits from HHS. In other words, food stamp applicants are asked about their citizenship.

If they refuse to answer, they are recorded as a non-citizen. If they say they are a citizen, they are then asked for proof — unlike in the decennial and other surveys. They can produce some official documents and the surveyors don’t usually question the authenticity of the paperwork.

But they can be prosecuted if they are found to be committing a fraud.

Under Section B of Virginia’s application, for instance, there’s a question about “household composition” that asks marital status, highest education level, whether you are a veteran and “are you a U.S. citizen?”

“If no, immigration status,” the form asks, along with a lot of other things.

I’m told that non-citizens can’t get many of the benefits from HHS, but the children of non-citizens can. And the payments to the children are made to the parents, so there is little reason to lie about citizen status— except that mom or dad’s portion of the payment will be withheld.

My point is: This is yet another way for the Trump administration to determine how many non-citizens are living in an area of the country.

But all of the ways the Trump administration will try to determine citizenship probably aren’t going to be as accurate as just asking that simple question at the end of the decennial census next year.

Wise up, Washington! This is going to get too freaky and could get troublesome for a lot of families.