By Ariane de Vogue, CNN Supreme Court Reporter

(CNN) -- Several conservative justices on Monday seemed to suggest they could avoid immediately ruling on whether President Donald Trump's attempt to exclude undocumented immigrants from being counted when seats in Congress are divvied up between the states next year is lawful.

In over an hour of telephonic arguments, the justices seemed extremely skeptical that in the waning days of the Trump administration, the government has the time to fulfill its stated goal, with Justice Samuel Alito saying it would be a "monumental task" to complete such a count by December 31.

Several states and immigrant rights group want the court to affirm a lower court decision and block the White House proposal.

The justices also seemed to suggest that if the government does not follow through on its plan to exclude all undocumented immigrants the challengers would not be able to show that they had the legal injury necessary to bring the case. The court could rule that the case is premature and sidestep a decision on the legality of Trump's plan -- a very narrow victory for the President.

Chief Justice John Roberts questioned whether a later challenge would be more appropriate. Justice Amy Coney Barrett, Trump's third Supreme Court nominee, seemed to agree. "Doesn't that cut in favor of waiting," she said.

Barrett at one point did seem to suggest that the early founders meant that all residents should be counted in the Census, so the idea of excluding undocumented immigrants from part of the count may not pass constitutional muster because Trump exceeded his authority.

But her conservative colleagues zeroed in more on the threshold issues than the President's legal authority. Justice Brett Kavanaugh noted that if the Trump administration ultimately chose not to exclude all undocumented immigrants, but only a subset of them, new legal challenges would arise. He said that the challengers and the government would be "right back here."

It's the second time in two terms that the justices took up the 2020 Census, having previously blocked Trump's attempt to add a citizenship question.

If Trump succeeds in the new case, it could mark the first time that the United States excludes undocumented immigrants when it counts individuals so that seats in the US House of Representatives can be apportioned and also will impact the distribution of federal funds.

"The 2020 Census is about power and money," said Thomas Wolf, senior counsel for the Brennan Center.

"These numbers will determine how seats in Congress are split up over the next 10 years which will determine the political power that communities will have for the next decade and will determine whether they receive their fair share of the $1.5 trillion that is annually distributed by the federal government," Wolf added.

Trump's efforts are complicated by the fact that Census officials have indicated that they are having difficulties processing census responses to produce the final count. If the numbers are transferred to the President after inauguration day on January 20, Trump's initiative is unlikely to be executed by the Biden administration and the case may be moot.

The Constitution provides that representatives "shall be apportioned" among the states counting the "whole number of persons in each State" every 10 years.

Congress has directed the Secretary of Commerce to conduct the census and submit the tabulation to the President by December 31. The President, in turn, transmits the number to Congress usually around the first week or so of January.

Trump issued a memo issued in July outlining a new policy mandating that undocumented immigrants be excluded from the apportionment base. It directed the Commerce secretary to actually submit two numbers to the President. The first is the census count. The second excludes undocumented immigrants. The government says that other administrative records could be used to determine individuals' immigration status.

Acting Solicitor General Jeffrey Wall on Monday urged the court to wipe away lower court opinion that went against the administration because it has yet to determine the exact number of individuals it might seek to exclude. He left open the possibility of a post apportionment challenge.

In court papers, Wall told the court that the "President is the ultimate decisionmaker" concerning the contents of the Census and that the law does not "expressly require" him to use the data submitted by the secretary, but instead, the data from the count can be combined with administrative records for apportionment.

In addition, the language from the Constitution and statutes "has long been understood to cover only a state's inhabitants," Wall argued in court papers. "As history, precedent, and structure indicate, the President need not treat all illegal aliens as inhabitants of the States and thereby allow their defiance of federal law to distort the allocation of the people's representatives," Wall said.

A group of states and local officials, as well as organizations represented by the American Civil Liberties Union, sued.

A three-judge court in the Southern District of New York blocked the White House policy.

"Throughout the Nation's history, the figures used to determine the apportionment of Congress have included every person residing in the United States at the time of the census, whether citizen or non-citizen and whether living here with legal status or without," the court held.

Trump's memo, the court stressed, violates "Congress's mandate to use the results of the census -- and only the results of the census -- in connection with the apportionment process."

Lawyers for the states told the justices that "nothing in the text or history of the Constitution or the Census Act suggests that Appellants may treat millions of people who undisputedly live here as if they were not here, solely because of their immigration status."

And the ACLU's Dale Ho, representing individual groups, argue that the three branches of government over "more than two centuries all point to a single conclusion: the Constitution requires counting all persons in each state in the apportionment base -- including undocumented immigrants."