When Commerce Secretary Wilbur Ross announced in March 2018 that he was adding a question about U.S. citizenship to the next full U.S. head count, it set off a firestorm of criticism — and a spate of legal challenges by cities, counties, states and immigrant-rights groups. One of those, brought by the New York Attorney General’s Office, prompted a federal judge to block the addition of the question in January. The government has appealed, taking the case to the U.S. Supreme Court.

1. Why the backlash?

With President Donald Trump’s heated rhetoric in the air, critics say a question about citizenship status on the 2020 U.S. census could scare immigrants and noncitizens away from filling out the once-a-decade household survey. That would skew the count, diluting the political power of those who didn’t respond. The Trump administration calls that a fever dream and says it needs to ask the question to help enforce Section 2 of the Voting Rights Act of 1965, which prohibits discrimination in election procedures.

2. Has the census asked about citizenship before?

Yes. The question “Is this person a citizen of the United States?” or something like it was part of the census as far back as 1820. But it came to be seen as less important as the waves of immigrants to U.S. shores receded, and it last appeared on the complete, decennial survey in 1950. In 1970, thanks to political pressure, the question returned on the long-form survey sent only to some households. From there it migrated to the annual American Community Survey, which replaced the long form in 2005.

3. What’s at stake?

Power. Census results are used to apportion U.S. congressional seats, divvy up the Electoral College votes that determine the winners of presidential elections and distribute billions of dollars a year in federal grants and aid to states and localities. Census-guided changes to the U.S. political map could give Democrats or Republicans an advantage for a decade or more.

4. What’s the legal issue?

Whether the administration acted on a legitimate need for information on the noncitizen population or on a desire to limit its political power. Ross, whose department includes the Census Bureau, initially said he added the question after a request from the Justice Department. Later he acknowledged that he had discussed the issue with immigration hawks including Kansas Secretary of State Kris Kobach, who was on the president’s disbanded voter-fraud commission, and former White House chief strategist Stephen Bannon before pressing the Justice Department on the question. In his ruling Jan. 15, U.S. District Judge Jesse Furman said Ross’s decision “fell short” on studying relevant evidence and coming to a conclusion supported by it, complying with procedures and laws and explaining the facts and reasons behind it. Federal judges in Maryland and San Francisco have since echoed Furman’s ruling in similar determinations.

5. What’s the case against asking the citizenship question?

That its real purpose is to discourage people who live in immigrant communities from participating in the survey for fear that federal agents might use their responses to target them or someone in their household, even if they are in the U.S. legally. In legal terms, opponents say the decision to add the question was “unconstitutional and arbitrary.”

6. What does the government say?

That the citizenship question will improve the accuracy of the count and that claims about political motivation are based on “unrelated innuendos.” As for how the question came to be, the U.S. says that internal discussion of such important matters is common and that the secretary of commerce has complete control of the format and content of the census. The administration points to a 1996 Supreme Court decision that unanimously upheld the Census Bureau’s decision not to statistically adjust its survey results. President Trump has criticized Democrats who are questioning his push to add a citizenship question.

7. Does the Census Bureau share the identities of noncitizens?

No. The bureau wouldn’t pass along the name and address of a noncitizen to immigration authorities, for example. That’s not to say this isn’t a real fear among some Hispanics and other minorities, such as Asians, whose households may have disproportionate numbers of noncitizens, says William Frey of the Brookings Institution, an expert on the census who isn’t involved in the case. Frey says 14 percent of the U.S. population lives in households with one or more noncitizens.

8. How will the issue be resolved?

By the Supreme Court. The case will be its first direct review of an administration initiative since the justices upheld the president’s executive order last year making the U.S. generally off-limits to residents of six countries, five of which are mostly Muslim. It thrusts Chief Justice John Roberts’s court into an intensely political fight that will affect the allocation of congressional seats and federal dollars. Because the Census Bureau says it needs to start printing questionnaires by this summer, the high court is hearing the case on an expedited basis and a ruling is likely by late June — although the bureau says it is on schedule no matter what the court decides.