Thanks to a VC commenter, I discovered that in August 1960, the Democrat-controlled Senate passed a resolution, S.RES. 334, “ Expressing the sense of the Senate that the president should not make recess appointments to the Supreme Court, except to prevent or end a breakdown in the administration of the Court’s business.” Each of President Eisenhower’s SCOTUS appointments had initially been a recess appointment who was later confirmed by the Senate, and the Democrats were apparently concerned that Ike would try to fill any last-minute vacancy that might arise with a recess appointment.