Last week Democrats took the U.S. House and thereby captured the subpoena power they’ve been craving to torment President Donald Trump for at least the next two years. But voters declined to deliver a full-throated repudiation of Mr. Trump and appear to have expanded the Republican majority in the Senate. Voters have also shown Mr. Trump the path to a 2020 re-election. And he’s about to gain a nearly perfect adversary.

Amid the celebrations surrounding dozens of new House seats, Democrats have also been expressing their ongoing...