The crisis isn’t too much polarization. It’s too little democracy.

Imagine that, four years ago, Donald Trump lost the presidential election by 2.9 million votes, but there was no Electoral College to weight the results in his favor. In January 2017, Hillary Clinton was inaugurated as president, and the Trumpist faction of the GOP was blamed for blowing an election Republicans could have won.

The GOP would have been locked out of presidential power for three straight terms, after winning the crucial popular vote only once since 1988. It might have lost the Supreme Court, too.

And so Republicans would...