United Steelworkers leaders on Thursday bashed a union letter-writing campaign in support of Nippon Steel’s $14.9 billion takeover of U.S. Steel as “intimidating” and “unlawful” as President Joe Biden is due to decide the deal’s fate. Union members in the Pittsburgh region were asked to sign a letter to Mr. Biden that states the merger with Japan’s largest steelmaker will “ensure that USW-represented facilities across the organization’s footprint will continue to mine, melt, and make steel...