Washington (AFP) - The right-leaning US Supreme Court appeared doubtful Monday about President Donald Trump's plan to exclude potentially millions of illegal immigrants from the national census count, which could shift the balance of US politics over the next decade.
But it was not clear if the nine justices would block Trump or let him follow through in the last weeks of his administration, likely sparking a messy and more prolonged court battle next year when he is out of office.
Trump, who has pursued a broad anti-immigrant policy over the last four years, has radically challenged the inclusion of undocumented immigrants in the census used for deciding how many seats each state hold in Congress.
The case heard Monday was likely the last of the Republican president's many challenges to constitutional law to rise to the high court.
After a mixed record, especially on his efforts to slash both legal and illegal immigration, Trump is hoping for one final win before he hands the White House over to Democrat Joe Biden on January 20.
Under constitutional law, anyone "resident" in the United States at the time of the census, including undocumented immigrants, is included in the decennial population count.
Trump is now seeking to exclude them when it comes to calculating each state's number of seats in the House of Representatives.
The current system has long given states like California and Texas, with large migrant populations, extra seats in the House.
According to the Pew Research Center, if undocumented immigrants are excluded, California, Florida and Texas could each lose a seat, while Minnesota, Alabama and Ohio would likely gain one.
Earlier this year lower courts ruled that Trump did not have the power to adjust the census totals by subtracting estimates of illegal immigrants.
Arguing for the Trump administration, Acting Solicitor General Jeffrey Wall called that simply a "historical practice" that can be broken.
"I'll certainly grant that no president has made this judgement before. No president has ever focused on it before," he told the court.
Wall admitted that it would be next to impossible to determine the total number of illegal immigrants counted by this year's census.
But he said certain categories, including those detained for expulsion by US Immigration and Customs Enforcement, and others registered under government programs, are known.
Dale Ho, a lawyer for the American Civil Liberties Union, told the court that undocumented immigrants have a deep role in US society and should all be counted.
"The president may have some discretion in borderline cases. He does not have authority to erase millions of state residents from the apportionment based solely on unlawful immigration status," Ho argued.
"Undocumented immigrants contribute $1 trillion in GDP, $20 billion in federal taxes," he said.
"One in four are homeowners and pay property taxes. They're our neighbors, our co-workers, our family members, and they are 'usual residents' under any plausible definition of that term."
Justices on both sides of the political spectrum -- three of them appointed by Trump -- acknowledged the complexities, such as including in the count a person who crosses the border from Mexico, is detained and is later expelled.
Such a person, Wall argued, certainly is not a settled resident of the United States.
But in their questions several justices suggested that changing the precedent, or even trying to refine it, is complicated, especially with just one month before the census numbers are scheduled to be reported to the president.
"If it's an undocumented person who has been in the country for say 20 years, even if illegally, why would such a person not have a settled residence here?" asked Amy Coney Barrett, the most recent justice appointed by Trump.
Wall argued that the lower court's rejection of Trump's plan was at any rate premature, because what categories will be deducted are not yet determined.
He suggested that the court could let Trump proceed, and entertain a challenge after the numbers are submitted for congressional apportionment early next month.
Chief Justice John Roberts said waiting until apportionment is decided would make the problem much more complicated, forcing the court to "unscramble the eggs."