For a flavor of politics in Washington next year, take a look at this final week of the lame-duck Congress. Republicans on Wednesday essentially agreed to sign letters of safe passage out of town, kicking spending and immigration issues into 2019 with a short-term continuing resolution to fund the government for two months.

The GOP rout was probably inevitable after President Trump volunteered last week to take credit for a government shutdown that would begin on the weekend before Christmas. He dared Democratic leaders Chuck...