President Donald Trump added more fuel to a brewing global trade battle Sunday, tweeting in support of his planned import tariffs on steel and aluminum to help U.S. industries.

“We are on the losing side of almost all trade deals. Our friends and enemies have taken advantage of the U.S. for many years. Our Steel and Aluminum industries are dead. Sorry, it’s time for a change! MAKE AMERICA GREAT AGAIN!” Trump wrote.

Trump last week said he would impose tariffs on imported steel to protect a U.S. industry that employs about 140,000 Americans. Still, analysts say that by raising the price of steel, those same tariffs stand to hurt a far larger group of U.S. workers: the 6.5 million who work in industries that buy steel — from automakers to aircraft manufacturers to suppliers of building materials.

Trump has vowed to this week impose 25 percent tariffs on imported steel and a 10 percent tariff on aluminum, in the interest of protecting America’s national security.

While his rhetoric has been focused on China, the duties also will cover significant imports from Canada, Mexico, South Korea, Japan and the European Union, officials said.

Trump’s announcement roiled markets, angered world leaders, rankled allies and raised prospects for a trade war.

The Dow Jones Industrial Average plunged 420 points on Thursday and an additional 71 points Friday, following Trump’s announcement.

Few issues could blur the lines of partisanship in Trump-era Washington. Trade is one of them.

Labor unions and liberal Democrats are in the unusual position of applauding Trump’s approach, while Republicans and an array of business groups are warning of dire economic and political consequences if he goes ahead with the tariffs.

Trade politics often cut along regional, rather than ideological, lines, as politicians reflect the interests of the hometown industries and workers whose wages have stagnated over the years.

As a candidate, Trump made his populist and protectionist positions on trade quite clear.

“This wave of globalization has wiped out totally, totally our middle class,” Trump told voters in the hard-hit steel town of Monessen, Pennsylvania, during one of his campaign stops. “It doesn’t have to be this way.”

Trump’s current GOP allies on Capitol Hill have little use for the tariff approach. They argue that other industries that rely on steel and aluminum products will suffer. The cost of new appliances, cars and buildings will rise if the president follows through, they warn, and other nations could retaliate. The end result could erode the president’s base of support with rural America and even the blue-collar workers the president says he trying to help.

“Sometimes a president needs to, you need to stick to your principles but you also need to recognize in cases where stuff you said in the campaign isn’t right and ought to be drawn back,” The Business Roundtable’s Josh Bolten, a chief of staff for President George W. Bush, said on “Fox News Sunday.” ‘‘The president needs to have the courage to do that.”

The Associated Press contributed to this report.