That is an interesting point. Of course, the fear from the Republican standpoint is if Obama nominates a liberal, then the court has a "liberal" majority. While this is true, it is something of a false argument. The problem for these people is, the court is always tipped one way or another because their are 9 justices - a majority of Justices are always appointed by either a Republican or a Democrat.
The problem this year, the Republicans are used to having the majority on the court -- their have been 5 justices appointed by a Republican President and only 4 by a Democrat for the last few decades. Essentially, the Republicans are having a temper tantrum that they aren't going to be able to keep that 5th Supreme Court justice.
I think more interesting, the oldest justices now seem to be "liberal." Ginsberg is the oldest at 82; Kennedy, while nominated by a Republican is a centrist and has been the swing vote for liberal wins, is 79 -- he's older than Scalia was, and Breyer is 77.
The real Republican plan here is to pack the court with Republicans. If a Republican wins the next election they expect Ginsberg is likely to either die or step down, she'll be 91 by the time the next president (assuming it is a Republican) leaves office. Breyer will be 86, it is hard to say if he will be alive or dead -- but the key is between Ginsberg and Breyer the odds are at least one will die (or be forced to retire). The idea here, if they can replace Scalia plus replace either Ginsberg or Breyer (or hopefully both), you then have a 6-3, or even 7-3, conservative advantage on the court -- particularly if they can replace Kennedy, who will be 88.
This whole thing about a "liberal" Supreme Court is largely a smoke screen and more about playing a long game. If the Republicans win the White House, the hope is that they can remake the Supreme Court in their image giving them a Republican majority that will last decades. If they lose in November they've lost little, other than Obama will likely nominate a centrist now, whereas Clinton/Sanders would likely nominate a justice that would have more liberal views -- but the Democrats still likely only have a 5-4 advantage.