Harley-Davidson's sales plunged by 13 percent last quarter after President Trump Donald John TrumpSteele Dossier sub-source was subject of FBI counterintelligence probe Pelosi slams Trump executive order on pre-existing conditions: It 'isn't worth the paper it's signed on' Trump 'no longer angry' at Romney because of Supreme Court stance MORE said he would support a boycott against the company.

The 13 percent drop between July and September is the steepest quarterly slide the company has seen in eight years, Bloomberg reported.

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The plunge followed after Trump tweeted his support in June for a boycott against Harley-Davidson after the company announced it would move some of its production overseas to avoid new tariffs imposed by the European Union.

The EU's motorcycle tariffs came in response to Trump's steep tariffs on steel and aluminum that were levied against the EU and other U.S. allies.

Harley-Davidson Inc. CEO Matt Levatich reportedly circulated a memo at the time to employees and dealers, emphasizing motorcycles sold in the U.S. will still be built in the U.S.

"The only reason we have invested otherwise is so that our products have a fighting chance of being price competitive in markets that burden our products with high tariffs," Levatich wrote in the June memo.

Trump's tweet backing a boycott of the company came in response to a report by The New York Times, in which some Harley-Davidson owners said they would abandon the company if it followed through with plans to shift some production overseas.

"Many @harleydavidson owners plan to boycott the company if manufacturing moves overseas," Trump tweeted. "Great! Most other companies are coming in our direction, including Harley competitors. A really bad move! U.S. will soon have a level playing field, or better."

Many @harleydavidson owners plan to boycott the company if manufacturing moves overseas. Great! Most other companies are coming in our direction, including Harley competitors. A really bad move! U.S. will soon have a level playing field, or better. — Donald J. Trump (@realDonaldTrump) August 12, 2018

Harley-Davidson is one of several companies that has vowed to move some production overseas in response to retaliatory tariffs.