Trump tweeted Thursday that he will be holding a press conference in the afternoon on the matter. He wrote that after the “very big and very important Social Media Summit,” he will head “to the beautiful Rose Garden for a News Conference on the Census and Citizenship.” 

Reports suggest that Trump will take executive action to include the citizenship question, despite the fact that the administration has already begun printing the census forms without it.

But it’s unclear if it is actually in Trump’s power to use executive action to force the question’s inclusion.

First, the responsibility for carrying out the census falls on Congress—not Trump.

Article I of the Constitution says that the census “shall be made within three years after the first meeting of the Congress of the United States, and within every subsequent term of 10 years, in such manner as [the House and Senate] shall by law direct.”

Congress, in turn, handed the power of conducting the census over to the commerce secretary in the Census Act. While the commerce secretary is appointed by the president and part of the executive, not legislative, branch, the laws and regulations of the Census Bureau are still subject to judicial review.

That is precisely what happened two weeks ago when the Supreme Court ruled against including the question, saying that the administration hadn’t provided adequate justification for it. The Court sent the matter back to the Commerce Department, leaving the door open to its eventual inclusion if the administration could come up with a new justification. 

Whether the administration chooses to take that route or executive order, the fact remains that the matter will likely need to be re-litigated. Even an executive order would in all likelihood yield another legal challenge—one which the administration may not have time to tackle with the printing deadline fast approaching.

A citizenship question is not unheard of as far as the U.S. census goes. From 1890 to 1950 the question was included on every census. In the years since, however, no citizenship question has appeared on the short form that most households receive. Citizenship data came instead from the long form questionnaire between 1970 and 2000. Today, the government derives this data from the American Community Survey, which is conducted every year, but is only sent to a small sample of the U.S. population.

More must-read stories from Fortune:

—New York opens door for Congress to investigate Trump’s state tax returns

—Black women voters will be central to the 2020 election, experts predict

—Activists worry about potential abuse of face scans for ICE

—Iran has surpassed the uranium enrichment limit. Here’s what that means

—The Maddow question: Should commentators moderate debates?

Get up to speed on your morning commute with Fortune’s CEO Daily newsletter.