The Supreme Court is taking up one of the term's most consequential cases, President Donald Trump's executive order on birthright citizenship declaring that children born to parents who are in the United States illegally or temporarily are not American citizens, and he was in the courtroom on Wednesday to attend the arguments.

The justices will hear Trump's appeal of a lower-court ruling from New Hampshire that struck down the citizenship restrictions, one of several courts that have blocked them. They have not taken effect anywhere in the country.

Trump is the first sitting president to attend oral arguments at the nation's highest court. Crowds watched from the sidewalks as his motorcade drove along Constitution and Independence Avenues, passing the Washington Monument and the National Mall on the way to the court building.

The case frames another test of Trump's assertions of executive power that defy long-standing precedent for a court that has largely ruled in the president's favor — but with some notable exceptions that Trump has responded to with starkly personal criticisms of the justices. A definitive ruling is expected by early summer.

The birthright citizenship order, which Trump signed the first day of his second term, is part of his Republican administration's broad immigration crackdown.

Birthright citizenship is the first Trump immigration-related policy to reach the court for a final ruling. The justices previously struck down global tariffs Trump had imposed under an emergency powers law that had never been used that way.

Trump reacted furiously to the late February tariffs decision, saying he was ashamed of the justices who ruled against him and calling them unpatriotic.

"Birthright Citizenship is not about rich people from China, and the rest of the World, who want their children, and hundreds of thousands more, FOR PAY, to ridiculously become citizens of the United States of America. It is about the BABIES OF SLAVES!," the president wrote. "Dumb Judges and Justices will not a great Country make!"

Trump's order would upend the long-standing view that the Constitution's 14th Amendment, ratified in 1868, and federal law since 1940 confer citizenship on everyone born on American soil, with narrow exceptions for the children of foreign diplomats and those born to a foreign occupying force.