WASHINGTON (Reuters) - Some lawmakers in the U.S. Congress have expressed dismay at President Donald Trump's threat to slap new tariffs on Canada, China and Mexico, but they have limited power to stop him.

WHICH BRANCH OF GOVERNMENT HAS TARIFF POWER? 

The U.S. Constitution gave Congress the power to set and regulate tariffs, but over the last 70 years the body has repeatedly passed laws handing that power over to the president, whether by citing national security concerns or responding to surges of imports threatening domestic industries, according to the non-partisan Congressional Research Service.

That smoothed the path for Trump's increased tariffs in his first administration - when duties paid on U.S. imports doubled - and for Trump's most recent moves. 

Free-trade Republicans and Democrats on Capitol Hill have bristled at these tariffs, with many questioning the president's authority. 

Retaliatory tariffs from the target countries could also squeeze lawmakers into an uncomfortable position between their home state industries and the president's agenda. 

CAN CONGRESS CHALLENGE TRUMP ON TARIFFS? 

Trump cited a national emergency regarding the flow of fentanyl into the country, as well as illegal migration, in ordering the new tariffs.

The National Emergencies Act of 1976 allows any one member of Congress to force a vote on a joint resolution to terminate the emergency, which, if passed, could force an end to the tariffs, said Greta Peisch, the former general counsel to the U.S. Trade Representative and former trade counsel to the U.S. Senate Finance committee.

WOULD THAT WORK?

This potential joint congressional resolution would only have the effect of law if passed by both chambers of Congress, which are controlled by Trump's fellow Republicans, and if the president were to sign it into law, which would require him to reject one of his own marquee campaign promises.

In this situation of a presidential veto, the legislative body's veto override power does not have an "expedited" process and the resolution would have to go through both chambers with two-thirds support, according to the Congressional Research Service.

Congress last passed a joint resolution to end a presidentially declared national emergency in 2023 to end the national emergency on the pandemic, but it has never done so in the context of tariffs, Peisch said.

However, in forcing a vote, opposition Democrats could force Republicans to take a public stand on tariffs and any economic toll they take.