Biden emphasizes importance of maintaining strong American steel companies and supporting steel workers.
Sale under review by Committee on Foreign Investment in the United States due to potential national security concerns.
Biden's stance on sale aligned with his focus on strengthening America's manufacturing base.
United Steelworkers union publicly opposes the deal, alleging violation of union contract terms.
Nippon Steel pledges to honor commitments between U.S. Steel and the United Steelworkers Union.
Biden seeks endorsements from prominent unions including the International Brotherhood of Teamsters and United Steelworkers.
Pennsylvania Senators Bob Casey and John Fetterman also oppose the sale, citing concerns for workers.