US Supreme Court justices questioned the legality behind a wide swath of Donald Trump's tariffs on Wednesday, as they began hearing a landmark case that could uphold -- or upend -- the president's economic agenda.

Billions of dollars in customs revenue and a key lever in Trump's trade wars are at stake, while the conservative-dominated court once again grapples with the Republican's attempts to expand presidential power.

The high court's nine justices are considering Trump's citing of emergency powers to impose so-called "reciprocal" tariffs on nearly every US trade partner, as well as levies targeting Mexico, Canada and China over their alleged roles in illicit drug flows.

Opponents argue that such broad tariffs are not permitted by the International Emergency Economic Powers Act (IEEP

See Full Page