Washington (AFP) - A sharply divided US Supreme Court heard arguments on Tuesday in a case about the 2020 census that could have high-stakes implications for the future balance of political power in the United States.

At the heart of the case is a bid by the administration of Republican President Donald Trump to ask respondents of the 2020 census whether they are US citizens.

Opponents of the citizenship question say this could potentially curtail Democratic representation in Congress since most non-citizens reside in states with Democratic majorities.

If non-citizens avoid census-takers for fear of running afoul of the immigration authorities, states where they live could lose federal funding and seats in the House of Representatives.

House seats are allocated on the basis of the census and the data also play a major role in how more than $675 billion in federal funds is spent.

The US Constitution mandates a census every 10 years with congressional representation apportioned based on the "whole numbers of persons in each state."

Trump, who has made stopping illegal immigration a hallmark of his presidency, has said that without the citizenship question the census "would be meaningless and a waste" of billions of dollars.

The five conservative justices on the nine-member Supreme Court -- two of whom were appointed by Trump -- appeared to be receptive on Tuesday to the administration's arguments.

The four liberal justices were skeptical, raising questions about the administration's motivations and citing studies that showed a citizenship question would lead to undercounting.

Commerce Secretary Wilbur Ross, whose department oversees the census, announced in March 2018 that for next year's national population count he intended to reintroduce the citizenship question abandoned more than 60 years ago.

Some 20 states including Democratic-led California and New York, as well as major cities like Chicago and San Francisco, filed legal actions against the move.

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Courts in New York and elsewhere ruled against including the citizenship question, leading the Trump administration to ask the nation's highest court to intervene.

- Ruling expected by June -

The case is given added urgency because census forms will need to be printed up this summer. The court is expected to rule by June.

Both sides acknowledged on Tuesday that including the citizenship question would have an impact on the number of respondents.

According to US Census Bureau experts, between 1.6 million and 6.5 million people -- many of Hispanic origin -- would decline to respond while others would lie.

"There's no doubt that people will respond less because of the census. That's been proven in study after study," said Justice Sonia Sotomayor.

Sotomayor and other liberal justices questioned why Ross chose to include the question when the experts said it would provide less reliable data.

"Why is asking a question better when you know that asking a question is going to result in lots of non-responses and in lots of false reporting?" asked Justice Elena Kagan.

"A secretary can deviate from his experts' recommendations and from his experts' bottom line conclusions," Kagan said. "But he needs reasons to do that."

Kagan said she was left with the impression "the secretary was shopping for a reason."

Ross has said he chose to add the citizenship question in response to a request from the Justice Department, which he said wanted to collect more specific data to help with enforcement of the Voting Rights Act election law.

But a federal judge dismissed that explanation as a "sham" and said Ross pressured his own staff to ask the Justice Department to request the citizenship question.

"Nobody doubts that there will be less people reported," said Sotomayor, adding that establishing resident numbers was the goal, not finding out how many were citizens.

Arguing for the Trump administration, Solicitor General Noel Francisco said a citizenship question "has been asked as part of the census in one form or another for nearly 200 years."

Francisco was cut short by Sotomayor who said the question was dropped in 1950 and was being reinstated by the Trump administration.

Brett Kavanaugh, a conservative justice named to the court by Trump, pointed out that many other countries ask a citizenship question and the United Nations recommends that it be done.