President Trump showed no sign Friday of backing off his shock announcement a day earlier that he would slap tariffs on aluminum and steel imports -- tweeting that “trade wars are good, and easy to win.”

Trump stunned the political world by announcing the move, a policy issue close to his heart for decades, during a listening session Thursday with representatives from the industries. He said he had decided on tariffs of 25 percent for steel and 10 percent for aluminum and would press ahead next week.

Republican lawmakers traditionally in favor of fewer trade barriers reacted warily to the move while the Dow Jones Industrial Average plummeted more than 400 points on Thursday. In early morning trading Friday, the Dow fell more than 300 points but later recovered, closing down approximately 70 points.

But on Friday, Trump tweeted a point-by-point defense of his protectionist stance -- and embraced the "trade war" his detractors say he's inviting.

“When a country (USA) is losing many billions of dollars on trade with virtually every country it does business with, trade wars are good, and easy to win. Example, when we are down $100 billion with a certain country and they get cute, don’t trade anymore-we win big. It’s easy!” Trump tweeted.

He added: "When a country Taxes our products coming in at, say, 50%, and we Tax the same product coming into our country at ZERO, not fair or smart. We will soon be starting RECIPROCAL TAXES so that we will charge the same thing as they charge us."

Trump reportedly pressed ahead with his trade measures despite a monthslong effort by some top advisers to persuade him not to.

Despite Trump’s cheerleading, a slew of GOP lawmakers predicted the move would backfire.

"Trade wars are never won. Trade wars are lost by both sides. Kooky 18th century protectionism will jack up prices on American families -- and will prompt retaliation from other countries," Sen. Ben Sasse, R-Neb., said in a statement Friday. "Make no mistake: If the President goes through with this, it will kill American jobs -- that's what every trade war ultimately does."

He quipped, "So much losing."

Even Republicans normally aligned with the president were nervous.

“Tariffs on steel and aluminum are a tax hike the American people don’t need and can’t afford,” Senate Finance Committee Chairman Orrin Hatch, R-Utah, said. “I encourage the president to carefully consider all of the implications of raising the cost of steel and aluminum on American manufacturers and consumers.”

A spokesman for House Speaker Paul Ryan said the speaker hopes Trump "will consider the unintended consequences of this idea and look at other approaches before moving forward."

The announcement met with a frosty reaction from U.S. allies as well.

“We are engaging with the U.S. on what this announcement means in practice,” a spokesman for the U.K. government said. “We have been clear that we are particularly concerned by any measures that would impact the U.K. steel and aluminium industries.”

Canada’s minister of foreign affairs said that trade restrictions on Canadian steel and aluminium would be “absolutely unacceptable.”

“Any restrictions would harm workers, the industry and manufacturers on both sides of the border,” Chrystia Freeland said in a statement.

“The steel and aluminum industry is highly integrated and supports critical North American manufacturing supply chains. The Canadian government will continue to make this point directly with the American administration at all levels.”

That hostility also reportedly has been flaring inside the White House. The New York Times reports that Gary Cohn, Trump’s top economic adviser, warned Chief of Staff John Kelly that he would resign should Trump actually implement the policy.

A source close to Cohn told Politico that the move was a “brutal blow” for Cohn that violated his core belief that protectionism is economically backward.

But White House Press Secretary Sarah Sanders said on "Fox & Friends" that the president was moving forward with the plan, noting that tariffs formed a central part of his campaign platform.

“That’s what he campaigned on. That’s what he is delivering on,” she said Friday.

Fox News’ Serafin Gomez and Catherine Herridge contributed to this report.