WASHINGTON, Nov. 30 (Reuters) — Conservative U.S. Supreme Court justices on Monday appeared reluctant to issue a ruling immediately blocking a plan by President Donald Trump's administration to exclude immigrants living in the United States illegally from the population totals used to allocate congressional districts to states.

The court's conservatives, who hold a 6-3 majority, signaled that such a ruling might be premature based on the administration's admission that it does not know how or if it will be able to implement the proposal, a facet of Trump's hardline policies on immigration being pursued in his final weeks in office.

A ruling tossing the current legal challenge to the plan would leave open the possibility of subsequent lawsuits if the administration does actually exclude certain subsets of immigrants from the population count formally given to Congress for House of Representatives districts to be apportioned among the 50 states.

"We don't know how many aliens will be excluded, we don't know what the effect of that will be on apportionment. All these questions would be resolved if we wait until the apportionment takes place. Why aren't we better advised to do that?" conservative Chief Justice John Roberts asked.

The justices were expected to decide the case on a expedited basis, with a ruling before the end of the year. Democrat Joe Biden is set to become president on Jan. 20.

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The challengers to Trump's July directive include various states led by New York, cities, counties and immigrant rights groups. They have argued that the Republican president's move could leave several million people uncounted and cause California, Texas and New Jersey to lose House seats.

Lawyers for the challengers urged the court not to toss out the lawsuit now, asking the justices to wait for a few weeks until more information is available on what data the Census Bureau intends to submit to the president.

House districts are based on a state's population count in the decennial national census, which was conducted this year.

The challengers have said Trump's plan would dilute the political clout of states with larger numbers of illegal immigrants, including heavily Democratic California, by undercounting their true populations and depriving them of House seats. If California loses House districts, that likely would mean Democrats lose House seats, benefiting Republicans.

There are an estimated 11 million immigrants living in the United States illegally. Until now, the government's practice was to count all people regardless of their citizenship or immigration status. The U.S. Constitution requires the apportionment of House seats to be based upon the "whole number of persons in each state."

Acting Solicitor General Jeffrey Wall told the justices that it is "very unlikely" the administration will have the required data to exclude all immigrants in the country illegally. Instead, it may propose excluding certain groups, such as those in federal detention, and the total number may not be high enough to affect apportionment, Wall said.

The challengers have argued that Trump's policy violates both the Constitution and the Census Act, a federal law that outlines how the census is conducted. Trump's lawyers said in court papers that he acted within his authority and that the challengers lacked the necessary legal standing to bring the case.

The justices wrestled with whether it is premature for the court to rule now when it is not clear whether the administration will be able to implement its plan.

Conservative Justice Samuel Alito, in questioning Wall, said that for the administration to exclude all of the illegal immigrants living in the United States from the population count "seems to me a monumental task."

"I would think you would be able to tell us whether that remains a realistic possibility at this point," Alito said.

After conservative Justice Amy Coney Barrett, a Trump appointee, asked about the fact that the government during the entire history of the United States has included illegal immigrants in the population count, Wall acknowledged that this was the case and added that this represented "the best argument for the other side."

Barrett told Wall that a lot of the historical evidence and longstanding government practice "really cuts against your position." Barrett also challenged Wall on the government's position that an immigrant in the country illegally cannot be considered an inhabitant for the purposes of apportionment.

A three-judge panel in New York ruled against the administration in September. Federal courts in California and Maryland have reached the same conclusion in other cases though one court in Washington ruled for Trump.

By statute, the president is due to send Congress a report in early January with the population of each of the states and their entitled number of House districts.

The census itself does not gather data on a person's citizenship or immigration status. Trump's administration would base its numbers on data gathered elsewhere, though it has not explained the methods being used.

(Reporting by Lawrence Hurley; Editing by Will Dunham)