Since the end of his first month in office, Mr. Trump’s job-approval average has never topped the 46.1% of the popular vote he received in 2016. This was just enough to earn him narrow victories in the Midwestern states that gave him his Electoral College majority, but it won’t be enough to repeat this feat in 2020.

Here’s why: In 2016, nearly 6% of the popular vote went to independent and third-party candidates, compared with an average of 2% in the four preceding elections after Ross Perot had left the scene. This year, with no well-known candidate on the Libertarian ticket, no challenge from the right or center to the Republican nominee, and only token left-wing opposition to the Democratic nominee, this 6% is likely to fall back to its usual level. Unless Mr. Trump can raise his vote share significantly above what he received four years ago, Mr. Biden will win a comfortable majority of both the popular and electoral votes.

A president who wins office without even a plurality of the popular vote would usually spend his first term trying to make new friends. Not Mr. Trump. His policies and personal style are calculated to preserve and intensify his base, whatever the consequences for other groups.