When the “Steele dossier” was first published a year ago, it looked like a bombshell. The document, drawn up by the British ex-spy Christopher Steele, contained salacious allegations against President Trump and suggested that Russia had helped him win the 2016 election. No one has been able to corroborate its charges, but Democrats continue to see the dossier as a road map for impeaching Mr. Trump. Republicans, on the other hand, point out that it was created as opposition research, leading them to see it as an elaborate partisan ploy.

There...