Calling President Trump’s rationale for his global reciprocal tariffs a “breathtaking assertion of power,” a group of small businesses challenging him in a David versus Goliath case in front of the Supreme Court say it is an illegal tax on Americans.

“Taxing Americans is an odd way to combat foreign threats,” a new brief filed at the Supreme Court on Monday in support of the case states, asking the justices to affirm lower court rulings that the tariffs are an overstep of the president’s authority.

The case claims Mr. Trump’s reliance on the 1977 International Emergency Economic Powers Act to impose his “Liberation Day” duties is a use of the law that lawmakers never intended and has not been used before.

The Constitution gives Congress the power to impose tariffs on imports but Congress has delegated its authority in recent decades by passing legislation allowing the executive branch to impose the levies in various circumstances.

“When Congress has delegated its taxing power, it has done so clearly,” a second brief filed against the tariffs states, calling them an illegal $3 trillion tax on Americans.

The group of Democrat-controlled states filed their own brief on Monday in support of overturning the tariffs claiming Mr. Trump has usurped the power of Congress.

“Congress, not the President, decides whether and how much to tax Americans who import goods from abroad. This Court should reject the President’s bid to seize that power for himself,” the brief states.

The Court of International Trade, an obscure panel that oversees trade cases, ruled against Mr. Trump, saying he does not have “unbounded authority” to impose across-the-board import taxes.

The United States Court of Appeals for the Federal Circuit upheld the decision and now the Supreme Court will have to decide on the separation of powers question that is at the heart of the case. A decision against the president would topple Mr. Trump’s economic policy.

Members of the Trump administration have claimed the country will suffer devastating economic consequences without the tariffs.

Solicitor General D. John Sauer said millions of jobs will be eliminated and the future of Social Security and Medicare will be threatened if money already collected through tariffs has to be refunded.

Secretary Scott Bessent said that the large sums already collected will make it more difficult for the Supreme Court justices to rule against Mr. Trump.

The Supreme Court is scheduled to hear arguments in the case on November 5.