President Joe Biden has blocked the nearly $15 billion proposed deal for Nippon Steel of Japan to purchase Pittsburgh-based U.S. Steel — affirming his earlier vow on the presidential campaign trail to prevent the acquisition of Steeltown USA’s most storied steel company.

The proposed deal kicked up an election year political maelstrom across America’s industrial heartland and blocking it drew a threat of litigation from Nippon Steel.

“We need major U.S. companies representing the major share of US steelmaking capacity to keep leading the fight on behalf of America’s national interests,” Biden said in a Friday morning statement.

Nippon Steel and U.S. Steel blasted the decision, saying in a joint statement that Biden blocking the deal “reflects a clear violation of due process and the law” in a process it said was “manipulated” to advance Biden’s political agenda.

It also insisted that Biden cited no credible evidence of the deal presenting a national security problem and suggested it will sue, saying “we are left with no choice but to take all appropriate action to protect our legal rights.”

Biden’s decision comes after the Committee on Foreign Investment in the United States, known as CFIUS, failed to reach consensus on the possible national security risks of the deal last month, and sent a long-awaited report on the merger to Biden. He had 15 days to reach a final decision.

The committee, chaired by Treasury Secretary Janet Yellen and made up of other Cabinet members, can recommend that the president block a transaction, and federal law gives the president that power.

A U.S. official familiar with the matter, who spoke on condition of anonymity, told The Associated Press last month that some federal agencies represented on the panel were skeptical that allowing a Japanese company to buy an American-owned steelmaker would create national security risks.

The decision comes just weeks before the Democratic president is set to leave office and could damage relations between the U.S. and Japan, which is America’s biggest ally in Asia and its largest foreign holder of U.S. debt.

In their statement, the two steel companies said it’s “shocking — and deeply troubling” that the U.S. would reject a transaction that advances U.S. interests and “treat an ally like Japan in this way.”

“Unfortunately, it sends a chilling message to any company based in a U.S. allied country contemplating significant investment in the United States,” the companies said.

Biden previously came out against the deal during the presidential campaign — and was backed by the United Steelworkers, concerned over whether the company would honor existing labor agreements or slash jobs, as well as over the firm’s financial transparency.

“It is important that we maintain strong American steel companies powered by American steel workers,” Biden said in March.

President-elect Donald Trump has also opposed the acquisition and vowed in December on his Truth Social platform to block the deal and to use tax incentives and tariffs to boost U.S. Steel’s fortunes.

On Friday, Steelworkers President David McCall said the union is grateful for Biden’s move to block the sale and, in a video news conference, called it the “right move for our members and for America’s national security.”

McCall had long questioned Nippon Steel’s status as an honest broker for U.S. national trade interests and reiterated that Friday, calling Nippon Steel a “serial trade cheater” that would degrade U.S. steelmaking and had, for decades, undermined the domestic steel industry by dumping its products into U.S. markets.