US Steel's stock price was down more than 8% in trading on Tuesday morning after President-elect Donald Trump repeated a vow to block the company's takeover by Nippon Steel.
Trump said he was "totally against" the plan.
"I will block this deal from happening," Trump said in a post on his Truth Social account. "Buyer Beware!!!"
Nippon's takeover of the embattled U.S. company is a deal worth $14.9 billion including debts.
US Steel has argued that it needs the deal to ensure its Pennsylvania plants can stay open.
That hasn't stopped Trump from claiming he will come up with a plan to save the company.
"Through a series of Tax Incentives and Tariffs, we will make U.S. Steel Strong and Great Again, and it will happen FAST! As President, I will block this deal from happening," Trump stated.
Nippon Steel said it expected to close its takeover of the company before President Joe Biden leaves office.
Biden has also voiced opposition to the deal.
He has said that it was "vital" for US Steel "to remain an American steel company that is domestically owned and operated."
The deal is being reviewed by a body helmed by Treasury Secretary Janet Yellen that reviews foreign takeovers of U.S. firms.
Major Japanese and American business groups have urged Yellen not to succumb to political pressure when reviewing the proposed acquisition, the AFP reported.
The steelworkers union is fighting the deal and criticized a ruling that Nippon had proven it could assume US Steel's labor contract obligations.
Some US Steel workers support the deal because they think it will help keep plants from shutting down.
