As President Donald Trump continues to advance his hard-line protectionist trade agenda, Republican senator Bob Corker is leading an effort to curtail the president’s ability to unilaterally impose sweeping tariffs on national security grounds.

Trump made use of Section 232 of the Trade Expansion Act in the spring to impose far-reaching tariffs of 25 percent and 10 percent on steel and aluminum, claiming that imports of those products threaten the national security of the United States. Last week, Trump expanded the tariffs to include U.S. allies and close trading partners like Canada, Mexico, and the European Union. Corker has been an outspoken critic of Trump’s strategy, arguing that Congress should reclaim its trade authorities under the Trump administration.

The Tennessee Republican said on Tuesday that his legislation could be introduced as soon as that afternoon. The bill would require approval from Congress within 60 days before the administration could set proposed Section 232 tariffs in place. CNN’s Phil Mattingly reports the bill would be retroactive for two years, meaning it would affect Trump’s steel and aluminum tariffs.