WASHINGTON — Robert Lighthizer just became one of the most powerful people in Washington.

Mr. Lighthizer, the United States trade representative, will spend the next 14 days deciding which countries, if any, will be exempt from the stiff and sweeping steel and aluminum tariffs that President Trump authorized on Thursday.

The issue is expected to dominate discussions in Brussels on Saturday between Mr. Lighthizer and his trade counterparts in Japan and the European Union, with the European trade commissioner saying on Thursday that Europe should be excluded. Dozens of other countries that import metals into the United States — such as Brazil, the United Arab Emirates and South Korea — are demanding a carveout or threatening retaliation if they are included.

It is a fight Mr. Lighthizer has been preparing for his entire life.

Mr. Lighthizer, 70, grew up in a well-off family in a Midwest town that did not share his fortune. Ashtabula, Ohio, a port town on the shores of Lake Erie, saw its surrounding steel mills shutter and decline as factories automated and moved abroad. The decimation of local manufacturing shaped his views on trade, friends said, hardening his approach and leading him to become a skeptic of globalization.

In the 1980s he brought that skepticism to Washington where, as deputy trade representative for President Ronald Reagan, he wielded the threat of tariffs to persuade Japan and other countries to cut their steel shipments to the United States. A lawyer by training, he continued his defense of American industry after his government stint, filing lawsuits on behalf of American steel giants like United States Steel who claimed they were withering from foreign competition and sought government protection from what they saw as unfair practices abroad.