A majority of Supreme Court justices seemed skeptical Wednesday about President Donald Trump ’s ability to unilaterally impose far-reaching tariffs, putting at risk a cornerstone of his agenda in the biggest legal test yet of his boundary-pushing presidency.

Three conservative justices raised questions about whether an emergency law gives Trump near-limitless power to set and change duties on imports, with potentially trillion-dollar implications for the global economy.

The court’s three liberal justices also appeared dubious, so at least two conservative votes could limit Trump’s tariff power under the law. It likely would not end it altogether, however.

The case is the first major piece of Trump’s agenda to come squarely before the nation’s highest court, which he helped shape by naming three of the nine justices in his first term.

The conservative majority has so far been reluctant to check his extraordinary flex of executive power in short-term orders in cases ranging from high-profile firings to major federal funding cuts.

That could change with a more detailed ruling in the tariff case, though it will likely take weeks or months to come down.

The Constitution says Congress has the power to levy tariffs. But, in a first, the Trump administration argues that an emergency law allowing the president to regulate importation also includes imposing tariffs.