By Makini Brice and Nick Brown

WASHINGTON/NEW YORK, June 27 (Reuters) - President Donald Trump on Thursday proposed an extraordinary delay to the 2020 census after the U.S. Supreme Court blocked his effort to add a citizenship question, but legal experts said such a move would violate the Constitution.

The U.S. Constitution directs the census take place every 10 years, and a delay via executive action would be unprecedented, experts said.

Terri Ann Lowenthal, a former staff director of the House of Representatives subcommittee overseeing the census, said a delay would run afoul of “the constitutional requirement for an enumeration every 10 years” and “the statutory requirement to … report the state population totals by the end of the census year.” “Essentially, we would be facing a constitutional crisis,” Lowenthal said.

On Twitter after his setback before the Supreme Court, Trump said, "I have asked the lawyers if they can delay the Census, no matter how long, until the United States Supreme Court is given additional information from which it can make a final and decisive decision on this very critical matter."

No citizenship question has been a part of the census since 1950.

Delaying the census by executive action would be “unprecedented,” said Margo Anderson, a U.S. census historian and professor at the University of Wisconsin at Milwaukee.

The census date of April 1 is written into the federal census act and would need to be changed legislatively for a delay to be constitutional, she said.

The Census bureau's own experts have estimated that about 6.5 million people would not respond to the census if the citizenship question were added.

The nation's top court said the federal government's rationale for the question "seems to have been contrived" and the government had not given a reasoned explanation for its actions.

The Supreme Court sent the issue back to the Commerce Department, which is in charge of the census, to decide whether to provide a different rationale for the question.

Officials say there is little time remaining to begin printing materials to meet the April 1 deadline for the census. The Census Bureau has said that it must begin printing forms by June 30.

The census is mandated under Article One, Section Two of the U.S. Constitution. (Reporting by Makini Brice in Washington and Nick Brown in New York; addititonal reporting by David Morgan; editing by Kevin Drawbaugh and Cynthia Osterman)