When President Trump talks trade, it’s often about fighting to get back the jobs the United States has lost. He focuses on sweeping measures, such as rewriting the free-trade agreement with Canada and Mexico, or new taxes to make imports more expensive and boost competing American goods.

The bold actions Trump has proposed could succeed in preserving jobs or bringing them back to American shores. However, they could also end up having unintended negative consequences, economists say. Barriers to imports, for example, could cut into the profits of American manufacturers by raising prices on parts and supplies. They could spark retaliation from other countries and even a harmful trade war, where countries take turns hiking restrictions to undercut each other’s goods and services, raising prices for consumers in the process.

[Trump’s administration has a new target on trade — and it’s not China or Mexico]

The effect of a trade war on U.S. communities could be significant and widespread, according to research from the Brookings Institution’s Metropolitan Policy Program. Nearly 6 million U.S. jobs are directly tied to exports. Another 6 million are indirectly tied to trade — for example, the driver who transports a truck load of widgets to the port.

While exposure to trade differs substantially across the country, every U.S. metro area has some exposure, with at least $1 out of every $20 generated in the local economy coming from exports. Most cities get much more. That means these cities have a lot to gain as well as potentially lose from major changes to trade policy, said Joseph Parilla, a fellow at the Brookings Institution’s Metropolitan Policy Program.

[Trump says making products in the U.S. is ‘gonna be cheaper.’ Economists disagree.]