President Trump’s proposed tariffs on steel and aluminum imports horrify Republican leaders in Congress — but they could yet pay political dividends. Trump’s move, cast by the White House as an effort to protect American manufacturing, has significant appeal in the Rust Belt states that were pivotal to his shock 2016 election win. Some Democrats in the region worry that senior figures in their party, especially those whose bases are in affluent coastal cities, are underestimating the political potency of Trump’s announcement. “It worries me as a Democrat that the national Democrats don’t see that Pennsylvania, Ohio, Michigan, Iowa and...