President Trump signed proclamations for new steel and aluminum tariffs at the White House Thursday afternoon, despite intense opposition from his own party including both of Alaska's Republican senators.

The tariffs — 25 percent on steel imports and 10 percent on aluminum imports — will go into effect 15 days from now, and, according to the Associated Press, all countries will be invited to attempt to negotiate individual exclusions from the tariffs. 

"It's really an assault on our country. It's been an assault," Mr. Trump said of what he deems steel dumping on American soil, surrounded by aluminum and steel workers in the White House's Roosevelt Room.  

But there is still confusion over exactly what the president's proclamation ensures. A senior administration official told reporters Thursday afternoon that Mexico and Canada will be exempt for now, but Mr. Trump said they will be exempt if the North American Free Trade Agreement (NAFTA) is renegotiated successfully. In a meeting with his Cabinet earlier Thursday, the president said he reserves the right to change the figures and add and drop countries at will. Mr. Trump said his decision follows a nine-month investigation by his administration into the issue. 

"Many of the countries that treat us the worst on trade and on military are our allies as they like to call them. So we just want fairness. We just want fairness," the president said. 

Mr. Trump also suggested he wants to push "reciprocal" taxes with other nations.

"We're going to be doing a reciprocal tax program at some point," Mr. Trump said, mentioning India and China. "... It's a mirror tax. They charge us 50, we charge them 50."

The president said he wants to see a lot of steel coming into the country, but it needs to be "fair." The signing had some lighter moments. Mr. Trump told one worker present that his father was looking down on him proudly.

"He's still alive," the worker responded. 

Mr. Trump has long called for fairer trade deals, and argues that the tariffs are necessary to safeguard the nation's struggling steel and aluminum against less expensive foreign imports. 

But the president's party isn't on board. Republicans made a full-court press over the past 24 hours to try to change the president's mind on this issue, CBS News' Nancy Cordes reports.

On Thursday, Sen. Jeff Flake, R-Arizona, suggested he would introduce legislation to nullify anything the president issues on tariffs. 

Senator Orrin Hatch, R-Utah, who chairs the Senate Finance Committee, spoke to Mr. Trump. So did House Ways and Means Chairman Kevin Brady, R-Texas, who has been the GOP's point person on this issue in the House. Brady was trying to come up with an alternative plan the president could support; clearly, he was unsuccessful.

On Wednesday, more than 100 Republicans signed a letter to Mr. Trump, imploring him to hold off on the tariffs. Republicans are concerned the imports could offset the benefits of the tax overhaul they worked to hard to pass last year.

Alaska's senior U.S. senator, Sen. Lisa Murkowski, said in a Friday statement that she shared Trump's desire to rebuild the steel and aluminum industries, but had "significant concerns" about the tariffs' impact on the U.S. economy as well as its trade relationships and energy sector.

“A lot of good work has gone into ensuring that the U.S. is able to reclaim its role as a global energy leader," Murkowski said. "Higher prices for steel — which accounts for a significant portion of project costs — could easily set us back. There are more targeted steps we can take to preserve our relationships with our allies, avoid cost increases and retaliation, and yet still return steel and aluminum production to America.”

Alaska Sen. Dan Sullivan said Thursday that he had signed a letter to Trump along with six other GOP senators: Joni Ernst of Iowa, John Barrasso of Wyoming, Lamar Alexander of Tennessee, James Lankford of Oklahoma, Mike Rounds of South Dakota and Ron Johnson of Wisconsin. In it, the senators warn Trump that "imposing broad tariffs on both aluminum and steel could risk straining relationships with international allies and partners."

The president's top economic adviser, Gary Cohn, announced his impending resignation shortly after Mr. Trump announced the tariffs. 

The fact that the White House did agree to exempt Canada and Mexico from these tariffs – at least temporarily -- is not all that comforting to Republicans. They still worry about how other major U.S. allies all around the world will react.

"Just exempting NAFTA trading partners is not enough," one top GOP aide told Cordes.

They haven't given up on the possibility that they can get the president to tweak these tariffs if the markets or other countries react badly. Most Republicans say legislation blocking the tariffs is not an option – anything they pass would have to have enough votes to overcome a presidential veto, and there are too many Republicans who would be reluctant to vote against the president and too many rust belt Democrats who like the tariffs.

Republicans are especially frustrated because the White House has continued to keep them completely in the dark. In fact, as recently as 1 p.m., trade staffers in key congressional offices still had not been briefed on the White House tariff plan. 

Read more about this story on