This supreme Court pick should be postponed until after the 2018 election

Conservatives in Congress (Mitch McConnell) argued that the supreme Court vacancy under Obama should have been, and was, postponed a year because the 2016 election was seen as a referendum on that nomination. With conservatives just barely controlling the Senate, it is only logical that this supreme Court nominee should be postponed until after the 2018 election (just 4 months) as a referendum on this nomination. Is this argument logically sound? Does this argument fall apart because the previous referendum was uncalled for? If the previous referendum was legitimate, and there shouldn't be another referendum on this one, why, what false equivalency is there?