Markets plunge after Trump announces tariffs on steel, aluminum imports The president announced the tariffs surrounded by industry executives.

 -- President Donald Trump announced Thursday that the U.S. will impose tariffs on steel and aluminum imports starting next week, imposing a 10 percent tariff for aluminum and a 25 percent tariff for steel. The stock market nosedived and one GOP senator called the move "a big mistake."

"It'll be for a long period of time,” the president said of the coming tariffs, which he said are being written now and will be signed next week, pledging: "We’re going to build our steel industry back, we’re going to build our aluminum industry back."

The president made the announcement at a White House meeting with Secretary of Commerce Wilbur Ross and industry executives.

One aluminum company executive said his industry has lost jobs due to unfair competition that's led to a lack of investment.

“We need a level playing field or we are going to lose our manufacturing infrastructure and the national security issues that surround having a vibrant, capable manufacturing sector,” John Lapides, CEO of United Aluminum.

Wall Street's reaction was sharply negative with the Dow closing down more than 400 points, with traders concerned that a company like General Motors will have to pay more for the steel and aluminum in its cars and trucks, likely passing on increased costs to consumers.

And Trump's announcement faced an immediate backlash on Capitol Hill, notably coming from an unexpected quarter: his Republican allies on Capitol Hill.

Utah Republican Sen. Orrin Hatch, a long-time Trump ally and supporter, said the president should "carefully consider all of the implications."

“Tariffs on steel and aluminum are a tax hike the American people don’t need and can’t afford. I encourage the president to carefully consider all of the implications of raising the cost of steel and aluminum on American manufacturers and consumers," Hatch said in a statement.

Hatch, as chairman of the Senate Finance Committee, authored the legislation to overhaul the nation's tax code policy, which the GOP has touted as a tax break for the American people.

Sen. Ron Johnson, R-Wis., called the announcement "concerning."

"The president’s announcement that he plans to levy higher taxes on materials that Wisconsin’s manufacturers and consumers purchase is concerning. I plan to work with my colleagues and the administration to ensure fair, robust trade policies that protect our state’s workers and consumers," Johnson said in a statement.

Pennsylvania Republican Sen. Pat Toomey called the decision a "big mistake."

"Changing course from this approach by invoking national security as a means of imposing new, huge tariffs on all kinds of imported steel is a big mistake that will increase costs on American consumers, cost our country jobs, and invite retaliation from other countries," Toomey said in a statement.

Thursday’s announcement came after a morning of confusion at the White House about whether the announcement would occur at all. Just hours before the meeting, different White House officials offered conflicting guidance about whether the announcement was set to happen Thursday.

It follows a Commerce Department review of steel and aluminum imports and comes after the president signed executive actions in January approving new tariffs on imported washing machines and solar panels.

There are concerns that the move could spark a potential trade war, with other countries reportedly weighing whether to retaliate against the Trump administration's protectionist move.

Even as the president accused other countries of ruining the U.S. steel and aluminum industries, he made clear that he doesn’t blame those other countries for the current situation, but instead pointed to decades of U.S. trade policy.

“The fact is we weren't treated, and we haven't been treated fairly, by other countries. But I don't blame the other countries,” Trump said. “When I was in China, I said: 'Listen President Xi', I said, 'I don't blame you if you are able to get away with making almost $500 billion a year off our country, how could I blame you? Somebody agreed to these deals'.”

The president tweeted earlier on Thursday to complain that the industries have been “decimated” by unfair trade practices.

ABC News' Rebecca Jarvis contributed to this report.