Republicans in Congress, scrambling for cover from midterm headwinds that could sweep them from power, are struggling in a political environment dominated by the unpredictability of President Trump’s polarizing leadership.

As the first votes of the 2018 elections were cast Tuesday in Texas primaries, Trump blew up the Republicans’ carefully crafted strategy to run on tax reform’s economic expansion with unanticipated plans to levy steep tariffs on imported steel and aluminum.