It started when Trump imposed hefty tariffs on steel and aluminum imports earlier this year in an effort to bolster domestic manufacturing.
The President tweeted about the potential boycott on Sunday, reports CNN.
Trump is referring to a New York Times story published yesterday.
Trump accused Harley of using the European retaliatory tariffs as "an excuse" for moving manufacturing overseas.
"I'm riding my last Harley. I think that's too far-fetched an argument and we don't have to make it", Giuliani said. According to the same outlet, hundreds of thousands of motorcycle enthusiasts who attended the South Dakota event share this sentiment.
"Great!" he wrote of purported plans by customers of Harley-Davidson to boycott the venerable motorcycle company over its plan to move production of motorcycles sold in Europe to factories outside the U.S. The firm, founded in Milwaukee in 1903, estimated that it would lose $100 million annually from steel tariffs imposed by the president in March. But the company said in a regulatory filing that it doesn't plan to increase prices as it adjusts.
Trump has lashed out more than a half-dozen times against Harley-Davidson.
President Donald Trump stands in the rain with members of Bikers for Trump and supporters as they say the Pledge of Allegiance, Saturday, Aug. 11, 2018, at the clubhouse of Trump National Golf Club in Bedminster, N.J.
A Harley-Davidson spokesman didn't immediately respond to a voice mail and email seeking comment. Considering Donald Trump is, evidently refusing to back down, that may no longer be an option.
Last year, Harley-Davidson announced it would build a plant in Thailand after Trump pulled out of the Trans Pacific Partnership (TPP) trade deal, which would have abolished tariffs on their motorcycles across 40 percent of the world's economy.