He has publicly supported tariffs since the Eighties at the latest. Back then Japan was a major focus of his ire:
https://www.cbc.ca/news/canada/trump-tariff-history-1.7469877
Here’s some info from several articles:
‘Three decades before President Trump’s trade agenda jolted the world, he laid out his vision in full-page newspaper advertisements foreshadowing what was to come,’ writes Jacob M. Schlesinger of The New York Times in ‘Trump Forged His Ideas on Trade in the 1980s—and Never Deviated.’
“Japan and other nations have been taking advantage of the United States” for years, wrote the New York real-estate developer, in the typewritten letter addressed “To The American People,” his signature affixed to the bottom.’
“ ‘Tax’ these wealthy nations, not America. End our huge deficits, reduce our taxes…” the September 1987 ads demanded.’
“Let’s not let our great country be laughed at anymore.”
‘Asked in a recent Wall Street Journal interview about the origin of his views on trade, Mr. Trump said, “I just hate to see our country taken advantage of. I would see cars, you know, pour in from Japan by the millions.”
‘In the interview, Mr. Trump called Japan “interchangeable with China, interchangeable with other countries. But it’s all the same thing.”
‘Shortly after the 1987 publication of Mr. Trump’s book, “The Art of the Deal,” he applied his world view in speeches and television interviews to a raging trade debate as Japan flooded the U.S. with inexpensive, high-quality autos and electronics.’
‘He continued gaining attention, and the book became a best seller.’
'He followed his newspaper ads—they ran in the New York Times, Washington Post and Boston Globe—and a brief flirtation with the 1988 presidential campaign with appearances on talk shows, telling hosts such as Larry King and Oprah Winfrey:’
“I do get tired of seeing the country ripped off.” He told Diane Sawyer in 1989 he would impose a 15% to 20% tariff on Japanese imports, adding: “I’m not afraid of a trade war.”
‘He complained specifically about the persistent trade deficit with Japan costing the U.S. money, as well as Japanese “import quotas and tariffs to protect their own interest,” as he put it in his 1990 book “Trump: Surviving at the Top.”