WASHINGTON — Nearly four weeks into a costly global trade war with no end in sight, President Donald Trump is facing a barrage of lawsuits from state officials, small businesses and even once-allied political groups, all contending that the president cannot sidestep Congress and tax virtually any import at levels to his liking.

The lawsuits carry great significance, not just because the tariffs have roiled financial markets and threatened to plunge the United States into a recession. The legal challenges also stand to test Trump’s claims of expansive presidential power, while illustrating the difficult calculation that his opponents face in deciding whether to fight back and risk retribution.

None of the lawsuits filed this month are supported by major business lobbying groups, even though many organizations — including the U.S. Chamber of Commerce and the Business Roundtable — have been sharply critical of the president’s tariffs and lobbied to lessen their impact.

Last week, Democratic attorneys general from a dozen states asked a federal judge to block many of Trump’s tariffs on grounds that they had “upended the constitutional order and brought chaos to the American economy.” California sued earlier this month, claiming that the president’s policies harmed its economy and budget.

At the heart of the legal wrangling is a 1970s law, the International Emergency Economic Powers Act, which enables the president to order trade embargoes, set sanctions and limit foreign investment to ward off adversaries abroad.

Trump invoked that law to impose his initial duties on Chinese exports, in what he described as an effort to stop the flow of fentanyl into the United States. He also used those powers to establish a 10% tax on exports from nearly every other country and to justify what he calls “reciprocal” tariffs, which will charge even steeper duties on countries including U.S. allies. For evidence of an emergency, Trump primarily pointed to the trade deficit — the difference between what the United States exports to other nations and what it imports.

No president before Trump had ever imposed such import taxes under the emergency law, which does not mention the word “tariff.” That omission has set the stage for a series of legal clashes, hinging in part on whether the law truly empowers the president “without actually, explicitly, saying tariffs,” said Ted Murphy, a co-leader of the global arbitration, trade and advocacy practice at the law firm Sidley Austin.

The latest lawsuit arrived Thursday from the Pacific Legal Foundation, a group with reported ties to conservative donor Charles Koch. On behalf of a clothing company, a board game designer and other small businesses, the group faulted Trump for imposing an “unlawful and unconstitutional” 145% tariff on Chinese goods, resulting in higher prices for U.S. businesses.

Jamey Stegmaier, a co-founder of Stonemaier Games and a plaintiff in the case, said his company had more than 250,000 board games and other products on order that it could not easily import from China unless it was willing to pay a “total tariff tax of around $1.5 million.”