President Joe Biden is preparing to announce that he will block Japan’s Nippon Steel from acquiring U.S. Steel, according to a person familiar with the matter.The move would be a forceful rebuke of the nearly $15 billion merger, which has become embroiled in politics ahead of the election.The Committee on Foreign Investment in the United States has been examining the proposed acquisition over national security risks.A White House official told ABC News, “CFIUS hasn’t transmitted a recommendation to the President, and that’s the next step in this process.”The timing of an announcement from Biden formally blocking the deal is not yet clear, the person familiar said.Shares of U.S. Steel plunged more than 17% in trading Wednesday on the news, which the Washington Post was first to report.Biden previously outlined his opposition to the merger in March, saying, “It is important that we maintain strong American steel companies powered by American steel workers.” Former President Donald Trump has said he would block the deal. Vice President Kamala Harris added this week that she also opposes it.Presidents including Biden and Trump have blocked other business deals citing national security concerns through CFIUS — but this is unusual because Japan is such a close ally of the U.S.In a statement to ABC News, U.S. Steel said it had “not received any update or executive order in relation to the CFIUS process.”“We continue to stand by the fact that there are no national security issues associated with this transaction, as Japan is one of our most staunch allies. We fully expect to pursue all possible options under the law to ensure this transaction, which is best (for the) future for Pennsylvania, American steelmaking, and all of our stakeholders, closes,” the statement said.