Again and again since President Donald Trump returned to the White House, the Supreme Court's conservative majority has blessed his boundary-pushing policies, allowing them to take effect on an interim basis while litigation plays out in the lower courts.
But on Wednesday, the justices will consider for the first time whether to say "no" to Trump in a lasting way.
At issue is the legality of his signature economic policy: the use of emergency powers to impose sweeping tariffs on nearly every US trading partner. The outcome of the case could significantly affect the global economy, American businesses and consumers.
Experts say the case is a tossup that poses difficult legal and political considerations for the justices, made all the more tense by Trump's efforts to personalise the dispute.
Trump had mused about attending the court's argument this week, and has spoken repeatedly of the case's importance to him. On Sunday, he said he decided against going to court, but stressed that he considered the stakes to be monumental.
The case "is one of the most important in the history of the country", Trump wrote in a post on Truth Social. "If a President was not able to quickly and nimbly use the power of tariffs, we would be defenceless, leading perhaps even to the ruination of our nation."
Observers of the court said the justices would be keenly aware that Trump would perceive a legal defeat as a personal blow.
"You can't help but think that that’s going to be hovering over the decision-making process in this case," said Donald B. Verrilli Jr, who was the solicitor general during the Obama administration.
The tariffs case is the first time the justices have weighed the underlying legal merits of a key administration priority in Trump's second term.