President Joe Biden is expected to soon release a statement of concern about Nippon Steel Corp.’s proposed purchase of United States Steel Corp., people familiar with the matter say. Shares of the Pittsburgh-based company fell the most in almost four years.

U.S. Steel dropped as much as 15% to $39.86, the biggest intraday loss since June 2020. The move is the latest show of support from the Biden administration for the steel union as it seeks to win better terms from the Japanese steelmaker. Nippon’s proposal was to buy the company for $55 per share in cash.

The scope of Biden’s statement, and how far he’ll go, isn’t clear, said the people, who asked not to be named because the information is private.

The White House declined to comment. Nippon Steel and U.S. Steel didn’t respond to requests for comment. The news was first reported by the Financial Times.

Biden’s administration has regularly sent warnings about the deal and has steadfastly sided with the United Steelworkers over the sale of a once-iconic firm based in Pennsylvania, a crucial swing state. So far, the White House has stopped short of calling for the deal to be blocked.

Politics hang over the administration’s considerations, with Donald Trump pledging to block the deal outright.

The Nippon bid for U.S. Steel is up for review by the Committee on Foreign Investment in the United States, or CFIUS, a process shrouded in secrecy. Allies of Biden have urged the administration to kill the deal over national security concerns — despite Japan being a close ally — and the threat to unionized steel jobs.

The deal has also been blasted byUnited Steelworkers, with the union leader, David McCall, saying in interviews with Bloomberg News that he’s been unhappy with the potential takeover.

McCall and his top aides met with representatives from Nippon Steel last week. In February, the union said in a statement it had received “personal assurances” that Biden has “our backs.”

White House National Economic Council Director Lael Brainard said in December, days after the deal was announced, that the administration had concerns. The deal “appears to deserve serious scrutiny in terms of its potential impact on national security and supply chain reliability,” Brainard said at the time.

Members of Congress in both parties have also raised national security concerns, including because of the Japanese steelmaker’s exposure to China. And Trump has said that he would block Nippon’s takeover “instantaneously.”

Biden has staunchly supported labor unions throughout his presidency and is seeking the support of the unions and their members in his rematch with Trump.

U.S. Steel said in a government filing this week that it will hold a special shareholder meeting over the deal on April 12.

After a meeting with Nippon Steel last week, the union said the discussion yielded no progress and ended in less than an hour. Nippon said in a statement at the time that it addressed the union’s concerns about capital investment, job security and technology sharing.