President-elect Donald J. Trump on Tuesday picked Jamieson Greer, a lawyer and former Trump official, to serve as his top trade negotiator. The position will be crucial to Mr. Trump’s plans of issuing hefty tariffs on foreign products and rewriting the rules of trade in America’s favor.

Mr. Greer is a partner in international trade at the law firm King & Spalding. During Mr. Trump’s first term, Mr. Greer served as chief of staff to Robert E. Lighthizer, the trade representative at the time. He was involved in the Trump administration’s trade negotiations with China, as well as the renegotiation of the North American Free Trade Agreement with Canada and Mexico.

Before that, Mr. Greer served in the Air Force, where he was a lawyer who prosecuted and defended U.S. airmen in criminal investigations. He was deployed to Iraq.

“Jamieson will focus the Office of the U.S. Trade Representative on reining in the Country’s massive Trade Deficit, defending American Manufacturing, Agriculture, and Services, and opening up Export Markets everywhere,” Mr. Trump said.

The position of trade representative has historically been fairly low profile, but it has taken on greater importance under Mr. Trump. In his first term, the office helped wage a trade war against China, imposed substantial tariffs on its products and negotiated a series of trade deals.

In his next term, Mr. Trump has promised to again make aggressive use of the government’s authority over trade. On Monday, he said he would impose tariffs on all products coming into the United States from Canada, Mexico and China on his first day in office.

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