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US Steel's Clairton Coke Works facility in Clairton, Pennsylvania, on September 9.
CNN  — 

President-elect Donald Trump has again indicated that he intends to block a Japanese firm’s $15 billion takeover bid of US Steel upon taking office, saying the former American giant can eventually stand on its own.

“Through a series of Tax Incentives and Tariffs, we will make U.S. Steel Strong and Great Again,” he said on his Truth Social platform on Monday. “As President, I will block this deal from happening. Buyer Beware!!”

The intended buyer, Nippon Steel, said it was determined to protect and grow US Steel.

“We will invest no less than $2.7 billion into its unionized facilities, introduce our world-class technological innovation, and secure union jobs so that American steelworkers at US Steel can manufacture the most advanced steel products for American customers,” it said in a statement sent to CNN.

The United Steelworkers union, which endorsed Vice President Kamala Harris for president, said Tuesday it supported Trump’s opposition to the deal, writing in a statement: “The USW welcomes President Trump’s continued opposition to Nippon Steel’s acquisition of U.S. Steel, a deal with 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,” USW international president David McCall wrote.

The Pittsburgh-based company was formed in 1901 as a merger of the nation’s leading steel companies, including Carnegie Steel Corp, engineered by financiers JP Morgan and Charles Schwab. The new company became the world’s first to be valued at more than $1 billion.

But in recent years, US Steel has fallen far below other American steel companies in output and stock market value. And the US steel industry is a shell of its former self, with no company among the 10 largest steel producers around the globe.

In September, Nippon Steel was expected to re-file its application for a national security review by American regulators, buying Japan’s largest steelmaker another 90 days to close its acquisition of a US rival after political opposition emerged in an election year.

The surprise development allowed the steelmakers to reset the clock, potentially keeping the controversial deal alive. But Trump, then the Republican presidential nominee, as well as President Joe Biden and Harris, had all voiced their opposition.

The Committee on Foreign Investment in the United States (CFIUS), a team of Cabinet-level appointees that reviews all deals of a certain size involving US entities, had been studying the takeover of US Steel for several months on the grounds of national security.

With the White House signaling it would block the deal, and CFIUS officials wanting to operate outside the pressure of political calendars, an extension of the timeline seemed the best outcome, CNN has previously reported.

US Steel said in September it would be forced to lay off workers and shutter mills without Nippon’s backing. It had put itself up for sale last year after receiving an unsolicited $7 billion takeover offer from Ohio-based Cleveland Cliffs. The $14.9 billion offer from Nippon Steel resulted from that sale process.

— CNN’s Yumi Asada contributed reporting.