President-elect Donald Trump is threatening heavy taxes as retribution for U.S. companies that move their business operations overseas and still try to sell their product to Americans.

In a series of early-morning tweets Sunday, Trump vowed a 35 percent tax on products sold inside the U.S. by any business that fired American workers and built a new factory or plant in another country.

Trump campaigned on a vow to help American workers but also to reduce taxes and regulations on businesses.

Trump tweets "there will be a tax on our soon to be strong border of 35 percent for these companies wanting to sell their product, cars, A.C. units, etc., back across the border."

He says companies should be "forewarned prior to making a very expensive mistake."

The U.S. is going to substantialy reduce taxes and regulations on businesses, but any business that leaves our country for another country, — Donald J. Trump (@realDonaldTrump) December 4, 2016

fires its employees, builds a new factory or plant in the other country, and then thinks it will sell its product back into the U.S. ...... — Donald J. Trump (@realDonaldTrump) December 4, 2016

without retribution or consequence, is WRONG! There will be a tax on our soon to be strong border of 35% for these companies ...... — Donald J. Trump (@realDonaldTrump) December 4, 2016

wanting to sell their product, cars, A.C. units etc., back across the border. This tax will make leaving financially difficult, but..... — Donald J. Trump (@realDonaldTrump) December 4, 2016

these companies are able to move between all 50 states, with no tax or tariff being charged. Please be forewarned prior to making a very ... — Donald J. Trump (@realDonaldTrump) December 4, 2016

expensive mistake! THE UNITED STATES IS OPEN FOR BUSINESS — Donald J. Trump (@realDonaldTrump) December 4, 2016

Trump this week reached a deal with United Technologies Corp. to keep about 1,100 jobs at a Carrier air-conditioning plant in Indianapolis rather than move them to Mexico. The president-elect signaled he will frequently deal with corporate leaders to protect American workers.

“We’re going to have a lot of phone calls to companies that say they’re thinking about leaving this country, because they’re not leaving this country,” Trump said on Dec. 1 at the Carrier plant in Indiana.