Republicans, including House Speaker Paul Ryan, have expressed concern about President Donald Trump's planned tariffs.

They have the power to stop a trade war.

The president has tariff-setting authority only if Congress chooses to give it to him.

House Speaker Paul Ryan is very concerned about President Donald Trump's proposal to impose tariffs on steel and aluminum.

"The speaker is hoping the president will consider the unintended consequences of this idea and look at other approaches before moving forward," a Ryan spokesman told Reuters on Wednesday.

That was before Trump threatened to also impose a tariff on cars from the European Union.

Ryan doesn't have to hope. Congress has the power to stop the president from starting a trade war. The question is whether lawmakers will use that power, or whether they will continue to abdicate legislative responsibilities to the president.

Article I of the US Constitution vests the power to set tariffs in Congress. The president has the power to impose tariffs at his discretion only because Congress has passed laws granting him that power. If Republicans in Congress think Trump has a bunch of dumb, destructive ideas about trade, they should pass new laws that strip him of that power.

Trump is expected to justify his proposed metal tariffs by invoking a 1962 law that authorizes the president to impose tariffs unilaterally if doing so is necessary for "national security." Congress could pass new legislation that would require the president to seek a vote of Congress to certify his (bizarre) claim that our dependence on Canadian steel is a national security threat — and that would block his tariffs from going into effect absent such a vote.

Of course, Trump would most likely veto a bill to reduce his tariff powers, and there are some Democrats in Congress who agree with Trump about trade. But there are a lot of other Democrats who would relish the opportunity to reduce Trump's power and block one of his signature initiatives.

If Republicans in Congress are serious about free trade, they can most likely get enough Democratic votes to override a Trump veto of free-trade legislation.

So, what is Paul Ryan waiting for? He doesn't want Trump imposing new tariffs, and the Constitution gives Congress the power to stop him from imposing new tariffs.

So why doesn't he do something about it?