We don’t yet know whether President Trump delayed some military aid to Ukraine as leverage to get Ukraine’s president to reopen an investigation into Hunter Biden. But if we are concerned about U.S. officials inappropriately threatening aid to Ukraine, then there are others who have some explaining to do. It got almost no attention, but in May, CNN reported that Sens. Robert Menendez (D-N.J.), Richard J. Durbin (D-Ill.) and Patrick J. Leahy (D-Vt.) wrote a letter to Ukraine’s prosecutor general, Yuriy Lutsenko, expressing concern at the closing of four investigations they said were critical to the Mueller probe. In the...