Why does Trump want a citizenship question on the census? 1. Because he wants a fight . . . and . . . 2. the deplorables love the fight.

There was a time when Republicans claimed they cared about making sure the once-a-decade census remained true to its constitutionally mandated mission of providing an “actual Enumeration” of the nation’s population.

“Conducting the census is a vital constitutional obligation. It should be as solid, reliable and accurate as possible in every respect,” House Republican leader John A. Boehner (R-Ohio) declared back in 2009.

Conservatives also used to raise alarms that a nosy federal government would go beyond the narrow mission the framers spelled out for the census.

As preparations were underway for the 2010 count, then-Rep. Michele Bachmann (R-Minn.) announced: “I know for my family, the only question we will be answering is how many people are in our home. We won’t be answering any information beyond that, because the Constitution doesn’t require any information beyond that.”

That, of course, was when there was a Democratic president in the White House. “Everyone knows that it is possible to organize a Decennial Census in a way that benefits one party or another politically,” warned Bruce Chapman, who had been head of the Census Bureau under Ronald Reagan.

Now, those concerns appear to have evaporated, as Republicans rally behind President Trump and his shifting, incoherent rationale for demanding that a citizenship question be added to the 2020 Census.

They are doing that despite — or, perhaps, because of — the fact that asking people to declare their citizenship is likely to result in an undercount of immigrants and minorities. A recent study authored by several Census Bureau experts estimated that such a question would depress the overall response rate by slightly more than 2 percent; in other words, millions of people might simply disappear.

“I don’t understand why the president doesn’t have all the right in the world to ask that question,” Sen. Kevin Cramer (R-N.D.) told Fox Business on Monday. “When you think of all the questions we ask on a census, it seems like citizenship would be pretty basic, pretty important to know. It should add real value.”

That “real value”? Well, it’s not entirely clear.

An estimate of the overall number of noncitizens living in this country is already available, via the American Community Survey, a longer questionnaire that goes to about 3 million households a year and that does include a question about citizenship.

The Supreme Court did not buy Commerce Secretary Wilbur Ross’s initial argument, which was that the question was necessary to enforce the Voting Rights Act. Given that rebuke, the Commerce Department appeared ready to abandon the question, until Trump tweeted last week that the administration was “moving forward.”

What’s happened since then has only confused things more. The Justice Department has tried to swap out its team of lawyers working on the census court case, only to be told by an exasperated federal judge that it cannot until it first provides a satisfactory explanation for the change. Where Ross assured Congress that nothing in the census data would be used to enforce immigration law, Ken Cuccinelli, the acting director of U.S. Citizenship and Immigration Services, suggested it would help deal “with the burden of those who are not here legally.”

And Trump has thrown in a number of other rationales, none of which were among the initial reasons his administration cited. “Number one, you need it for Congress — you need it for Congress for districting,” he said. “You need it for appropriations — where are the funds going? How many people are there? Are they citizens? Are they not citizens? You need it for many reasons.”

The real reason, however, is probably the most obvious one. This is simply a fight that Trump wants to have. Most polling indicates that Americans are in favor of adding a citizenship question to the census, regardless of whether it skews the results.

Even more important to the president is the fact that this issue lights up his base. Trump’s reelection campaign has been sending a fundraising email telling supporters: “The American People deserve to know who is in this country and there’s only one way to find out: We need to ask every person ‘ARE YOU AN AMERICAN CITIZEN, YES OR NO?’ ”

Attorney General William P. Barr claims he and the president have been working on a way to “clarify the record in time to add the question” to the 2020 Census. But in fact, it is already pretty clear what is going on here.

Republicans who once warned that the census could be manipulated for political gain are now giving us a master class in how to do it.