WASHINGTON — After making good on tax cuts and regulatory rollbacks that business leaders wanted, President Trump has turned to a part of his economic agenda that many of them feared: tariffs.

Those leaders worry that Mr. Trump, by imposing stiff and sweeping tariffs on steel and aluminum, will set off a trade war with other countries. The global tit-for-tat could hurt American exporters and raise costs for manufacturers that rely on a vast supply chain around the world.

If that happens, it will crimp economic growth, undermining the stimulative effects of Mr. Trump’s deregulation push and his signature $1.5 trillion tax cut.

The odds of such an outcome now appear to be rising, prompting congressional Republicans to push Mr. Trump in public and in private to reconsider. “If the president goes through with this, it will kill American jobs — that’s what every trade war ultimately does,” Senator Ben Sasse, Republican of Nebraska, said on Friday. “So much losing.”