HARRISBURG, Pa. (AP) — President-elect Donald Trump is underscoring his intention to block the purchase of U.S. Steel by Japanese steelmaker Nippon Steel Corp., and he's pledging to use tax incentives and tariffs to strengthen the iconic American steelmaker.

Trump had vowed early in the presidential campaign that he would "instantaneously" block the deal, and he reiterated that sentiment in a post on his TruthSocial platform on Monday night.

"I am totally against the once great and powerful U.S. Steel being bought by a foreign company" and will use tax incentives and tariffs to make U.S. Steel "Strong and Great Again, and it will happen FAST!" he wrote.

"As President," he continued, "I will block this deal from happening. Buyer Beware!!!"

President Joe Biden, like Trump, also opposes Nippon Steel's purchase of Pittsburgh-based U.S. Steel. Biden's White House in September said that it had yet to see a report from the secretive Committee on Foreign Investment in the United States, which was reviewing the transaction for national security concerns. The committee can recommend that the president block a transaction, and federal law gives the president that power.

Nippon Steel has said it is the only company that can make the necessary investment in U.S. Steel's factories and strengthen the American steel industry. Both Nippon Steel and U.S. Steel on Tuesday released statements in support of the deal.

"This transaction should be approved on its merits. The benefits are overwhelmingly clear. Our communities, customers, investors, and employees strongly support this transaction, and we will continue to advocate for them and adherence to the rule of law," U.S. Steel said.

The deal follows a long stretch of protectionist U.S. tariffs that analysts say has helped reinvigorate domestic steel, including U.S. Steel. U.S. Steel's shareholders have approved the deal, but the United Steelworkers oppose it.

In a statement Tuesday, the union said the deal carries "serious long-term implications for U.S. economic and national security."

"It's clear that President Trump understands the vital role a strong domestic steel industry plays in our national security, as well as the importance of the jobs and communities the industry supports," the union said.

The deal has drawn bipartisan opposition in the U.S. Senate, including from the incoming vice president, Republican Sen. JD Vance of Ohio, although the federal government's objections to the deal have drawn criticism that the opposition is political.

Trump's statement came two weeks after Nippon Steel's vice chairman, Takahiro Mori, visited Pittsburgh and Washington to meet with lawmakers, local officials and workers in an ongoing persuasion campaign.

That campaign has included Nippon Steel's promises to boost its capital commitments beyond the original deal and, more recently, a pledge that it won't import steel slabs that would compete with U.S. Steel's blast furnaces.

Nippon Steel pledged in the deal to invest at least $1.4 billion in USW-represented facilities, not to conduct layoffs or plant closings during the term of the basic labor agreement, and to protect the best interests of U.S. Steel in trade matters.

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Boak reported from Washington.