Republican presidential frontrunner Donald Trump has made unfair foreign trade a central theme in his campaign for the White House, singling out U.S. companies such as Ford Motor Co. for moving operations abroad.Ford CEO Mark Fields said he wrote a letter to Trump defending his company, elaborating in an interview with CNBC : “Ford Motor Co. is here to stay in the United States. It's presidential politics and we are just going stay focused on facts.”Trump first blasted Ford in April when the carmaker announced a $2.5 billion expansion of operations in Mexico.At that time, Trump said he would call Fields and say: “Let me give you the bad news. Every car, and every truck and every part manufactured in this plant that comes across the border, we’re going to charge you a 35% tax. Okay? And that tax is going to be paid simultaneously with the transaction, and that’s it.”In his interview with CNBC, Fields said: “We're very proud as a company of what we do in terms of contributing to economic development here in the U.S. We invested over $10 billion since 2011 at our facilities. We hired 25,000 people with plans to hire another 8,500 folks. It's important for us to be successful in our home market and we love what we do for the economy.”It wasn’t the first time Fields responded to Trump, who accused the carmaker of backing off plans to expand U.S. operations. In an earnings call with investors in October , Fields said: “Facts are stubborn things, and at Ford we’re proud of the facts. Unfortunately, we suspect the facts are getting lost in the politics.”Trump has mentioned Ford throughout his presidential campaign, but that’s not the only company he’s singled out.In February, he also blasted air-conditioner maker Carrier, a unit of United Technologies Corp., after it announced plans to close a factory in Indianapolis and fire 1,400 employees.By moving the plan to Monterrey, Mexico, Carrier could pay employees $6 an hour instead of $34, according to a United Steelworkers representative cited by TheIndyChannel.com