The U.S. Supreme Court on Tuesday rejected President Donald Trump's audacious attempt to restrict birthright citizenship in the United States.

The 6-3 ruling marked the second time this year that the court has invalidated a major Trump initiative, following its February decision to strike down his sweeping global tariffs.

The justices upheld a lower court's decision that blocked Trump's executive order directing U.S. agencies not to recognize the citizenship of children born in the United States if neither parent is an American citizen or legal permanent resident, also called a "green card" holder.

Challengers to Trump's order argued that it violates language in the U.S. Constitution's 14th Amendment that confers citizenship to those born in the United States who are "subject to the jurisdiction thereof."

Trump, who has repeatedly tested the limits of presidential power in domestic and foreign policy, issued the order last year on his first day back in office as part of a suite of policies to crack down on legal and illegal immigration. Critics have accused the Republican president of racial and religious discrimination in his approach to immigration.

The Supreme Court weighs in on what it means to be an American citizen just ahead of the July 4 holiday when the United States marks the 250th anniversary of its founding.

Ahead of the ruling, some experts had estimated that Trump's directive could affect the legal status of as many as 250,000 ‌babies born each year and could require the families of millions more to prove the citizenship status of their newborns.