Boris Johnson’s big election victory this week drove another nail into the coffin of the brand of conservative politics Ronald Reagan and Margaret Thatcher first rode to power four decades ago.

As Mr. Johnson’s decisive win in a hotly contested national election illustrated, the conservative movement in the West now has become markedly more populist and nationalist, and appeals to a distinctly more working-class constituency. Fiscal restraint, once a cardinal tenet of conservatism, matters less; rewriting the rules that have...