Donald Trump’s surprise victory in last year’s presidential election has finally energized my fellow liberals, who are networking, marching and showing up at town-hall meetings across the country. There is excited talk about winning back the White House in 2020 and maybe even the House of Representatives in the interim.

But we are way ahead of ourselves—dangerously so. For a start, the presidency just isn’t what it used to be, certainly not for Democrats. In the last generation, Bill Clinton and Barack Obama won the office...