President Donald Trump threatened Thursday that if he’s re-elected, he’ll impose tariffs on U.S. companies that refuse to move jobs back to the country from overseas.

“We will give tax credits to companies to bring jobs back to America, and if they don’t do it, we will put tariffs on those companies, and they will have to pay us a lot of money,” Trump said during a campaign event in Pennsylvania.

“So what are they going to do? They are going to bring the jobs back,” Trump said.

He offered no explanation about how such a system of tax credits and tariffs would work, and it isn’t clear if the White House is developing such a policy.

“The beauty of the Trump tariffs is that they represent a powerful inducement for investment in domestic production by both American companies that have offshored our factories and foreign companies that want to sell into the U.S. market,” Trump trade adviser Peter Navarro said in a statement, without addressing whether a new policy is in the works.

“We have seen this lesson time and again during this administration with the steel and aluminum tariffs, with the threat of auto tariffs, and with the China tariffs,” he said. “Tariffs mean more American jobs and factories.”

Trump’s remarks came during a more than hourlong speech intended to rebut former Vice President Joe Biden’s acceptance of the Democratic nomination for president Thursday. Biden was born in Scranton, near the borough of Old Forge where Trump spoke.