His defiant plans to upend the country’s political system held appeal to tens of millions of voters who feared that the American dream was drifting further from reach and who turned to Mr. Trump as a battering ram against the ruling establishment and the expert class of elites.

In a deeply divided nation, voters embraced Mr. Trump’s pledge to seal the southern border by almost any means, to revive the economy with 19th-century-style tariffs that would restore American manufacturing and to lead a retreat from international entanglements and global conflict.