If American Democrats want a warning about the consequences of embracing the Green New Deal, look no further than Saturday’s election shocker in Australia. The opposition center-left Labor Party had led in the polls for months but lost as voters rejected its move left on taxes, spending and above all on climate change.

The ruling Liberal-National Coalition had been divided and tossed out two prime ministers during its nearly six years in power. Scott Morrison, the compromise choice as Prime Minister last year, managed to unite...