(Recasts first paragraph, adds Justice Kavanaugh)
By Andrew Chung and Lawrence Hurley
WASHINGTON, April 23 (Reuters) - The U.S. Supreme Court's conservative majority on Tuesday appeared inclined to hand President Donald Trump a victory on his administration's plan to add a citizenship question to the 2020 census, a move opponents call a Republican effort to deter immigrants from taking part.
During arguments in the closely watched case, conservative justices rallied in defense of the administration's stated justification for using the citizenship question in the decennial population count, while their liberal counterparts remained skeptical.
The court has a 5-4 conservative majority. Among the conservative justices indicating support toward the administration's stance were Trump's two appointees, Brett Kavanaugh and Neil Gorsuch, as well as Chief Justice John Roberts, considered the court's pivotal vote.
Lower courts have blocked the question, ruling that the administration violated federal law and the U.S. Constitution in seeking to include it on the census form. A ruling by the Supreme Court is due by the end of June.
Opponents have said inclusion of a citizenship question would cause a sizeable undercount by frightening immigrant households and Latinos from filling out the census forms, fearful that the information would be shared with law enforcement. This would cost Democratic-leaning areas electoral representation in Congress and federal aid, benefiting Trump's fellow Republicans and Republican-leaning parts of the country, they said.
The census is used to allot seats in the U.S. House of Representatives and distribute some $800 billion in federal funds.
During extended arguments that lasted about 80 minutes, Roberts and other conservative justices appeared to embrace the administration's argument that the question would yield better data to enforce the Voting Rights Act, which protects eligible voters from discrimination.
Roberts challenged New York Solicitor General Barbara Underwood, whose state sued the administration over the plan to add the question, saying citizenship is critical information for enforcing the Voting Rights Act. Roberts also said it is "quite common" for census questions to capture demographic information.
Kavanaugh cited other countries that ask a citizenship question as part of their censuses.
"It's a very common question internationally," Kavanaugh said.
Gorsuch and fellow conservative Justice Samuel Alito both challenged evidence that asking about citizenship could cause census response rates to decline, as the challengers contend. Gorsuch noted that "it's not like this question is improper to ask."
Trump has pursued hardline immigration policies. The Supreme Court already has handed him some major victories, including last year allowing his travel ban targeting people from several Muslim-majority countries.
The case comes in a pair of lawsuits by a group of states and localities led by New York state, and a coalition of immigrant rights groups challenging the legality of the question. The census forms are due to be printed in the coming months.
Liberal justices noted evidence presented in the case from the Census Bureau's own experts that showed the citizenship question would lead to a population undercount, and, contrary to the administration's stated goal, less accurate citizenship data.
They also appeared skeptical about the administration's stated justification regarding the Voting Rights Act.
"You can't read the record without sensing this need is a contrived one," Justice Elena Kagan said.
U.S. Solicitor General Noel Francisco, arguing on behalf of the administration, said Commerce Secretary Wilbur Ross, whose department includes the Census Bureau, acted within his discretion in deciding to add the citizenship question.
"It boils down to whether the secretary's judgment is a reasonable one," Francisco said.
Business groups and corporations such as Lyft, Inc, Box, Inc, Levi Strauss & Co and Uber Technologies Inc also opposed the citizenship question, saying it would compromise census data that they use to make decisions including where to put new locations and how to market products.
Manhattan-based U.S. District Judge Jesse Furman on Jan. 15 ruled that the Commerce Department's decision to add the question violated a federal law called the Administrative Procedure Act. Federal judges in Maryland and California also prohibited the question's inclusion in subsequent rulings, saying it would violate the Constitution's mandate to enumerate the population every 10 years.
In November, when the Supreme Court allowed the trial before Furman to proceed, three of the court's conservative justices - Gorsuch, Clarence Thomas and Samuel Alito - said they would have blocked it, indicating they may be sympathetic to the administration's legal arguments.
Furman found that Ross concealed his true motives for his March 2018 decision to add the question.
The Census Bureau itself estimated that households corresponding to 6.5 million people would not respond to the census if the citizenship question is asked, leading to less accurate citizenship data.
Citizenship has not been asked of all households since the 1950 census. It has featured since then on questionnaires sent to a smaller subset of the population. While only U.S. citizens can vote, non-citizens comprise an estimated 7 percent of the population.
For a graphic on the major Supreme Court cases this term: tmsnrt.rs/2V2T0Uf
Reporting by Andrew Chung and Lawrence Hurley; Additional reporting by Nick Brown; Editing by Will Dunham