TOKYO — President-elect Donald Trump said Monday that he would block a Japanese company’s $15 billion bid to acquire U.S. Steel, a contentious deal that faces widespread political opposition.

Nippon Steel, Japan’s largest steelmaker, announced the deal last December and has said it is determined to see it through despite concerns about the implications for employees, supply chains and U.S. national security.

“I am totally against the once great and powerful U.S. Steel being bought by a…