The United Steelworkers called on the Trump administration Friday to exempt Canada from steel and aluminum tariffs.

The union asked the White House to reinstate the exemption it had given Canada from the tariffs, which were announced in March. The union, which has backed the administration on many trade issues, called lifting the exemption against the U.S.'s northern neighbor "wrongheaded" because it would hurt the industry in both countries.

"This decision is unacceptable and calls into serious question the design and direction of the administration’s trade policy. Section 232 relief is founded on national security interests and U.S. law. Our history shows that there is no stronger ally and partner on national security than Canada," said the union, which represents 850,000 workers in the U.S. and Canada.

[Click here to read Canada's proposed retaliatory tariffs]

The union said lifting the exemption made no sense because Canadian exports are "fairly traded" and that Canada has cooperated with the U.S. on steel trade issues in the past. Section 232 refers to the national security section of the Trade Expansion Act used by the administration to justify the tariffs.

The United Steelworkers had been one of the loudest supporters of the administration's tariffs. Union President Leo Gerard said in March, when the tariffs were announced, "President Trump initiated action to investigate the impact of aluminum and steel imports on our national security. We applauded his action then and appreciate the attention that this issue has received in recent months."

The union now says it no longer understands the administration's reasoning: "In recent days, it has become increasingly difficult to understand the reasoning behind certain decisions and policies. The regular chaos surrounding our flawed trade policies is undermining the ability to project a reasoned course and ensure that we can improve domestic production and employment."

On Friday U.S. tariffs of 25 percent on imported steel and 10 percent on imported aluminum went into effect against Canada, Mexico and the European Union. The trading partners had been exempted from the tariffs as a temporary measure to see if the U.S. could resolve various trade disputes with them. After several months of talks with no progress, the U.S. announced Thursday the exemptions would lifted.

Canadian Foreign Affairs Minister Chrystia Freeland called the tariffs "unprecented and wrong" Thursday and said the U.S. would be hit with $12.8 billion in retaliatory tariffs.