I didn't want to say anything in my tribute to Justice Scalia about the politics of filling the vacancy he left behind. However, for what it is worth, I believe that the Senate should hold hearings and confirm Judge Merrick Garland to the Supreme Court.
Mind you, the Senate has NO constitutional obligation to consider any presidential nominee. The Constitution says only that "The President shall nominate, and, by and with the advice and consent of the Senate, shall appoint..." judges.
The phrase "advise and consent" cannot possibly be regarded as mandatory, since that would imply that the Senate must consent, which is absurd since nobody questions their power to reject a candidate by voting them down. Journalists and politicians who claim that the Constitution requires the Senate to "advise and consent" are abusing the meaning of these words. The word "consent" cannot possibly mean "decide whether to consent". And if the "consent" part is optional for the Senate, then the "advise" part must also optional for the Senate. Nor can "advise" mean "hold hearings", since the Senate didn't regularly hold hearings on Supreme Court nominees until recently. The phrase means only that IF the Senate provides its advice and consent, THEN the President may go ahead and appoint the nominee.
At best, the word "shall" imposes a duty on the President to nominate candidates. However, the Supreme Court has rejected this interpretation, saying that filling vacancies is also optional for the President (Marbury vs. Madison)!
Nevertheless, just because there is no constitutional obligation to confirm the nominee, doesn't mean the Senate is justified in its obstruction. There are other norms and conventions in politics besides constitutional norms. Without them, the system could not function.
For the past century or so, until very recently, there has been an expectation that the Senate will confirm any reasonable and moderate Supreme Court nominee proposed by the President. Voting down, and even filibusters, have been discussed only in cases where (it has been claimed that) the candidate is unqualified, extreme or unacceptable in some other specific way. And nobody has suggested that Garland is particularly extreme. This is not to say he is a conservative, but he does seem to believe in some sort of policy of judicial restraint, which is as good as Republicans can expect from a Democratic president.
At the "object level" (as opposed to the "meta level" of the politics of filling vacancies) I would very much prefer for Scalia to be replaced by someone with a similar judicial philosophy. However, the short term gain that comes from subverting the process for one nominee, will simply make it harder for Republicans to confirm nominees in the future. No one gains from increasingly bitter "no holds barred" confirmation fights. It's a game of interated Prisoner's dilemma, and both sides keep defecting!
Although pre-emptively announcing that one will not fill a vacancy is unprecedented obstruction in the modern era, one-up-manship in confirmation fights isn't at all unprecedented. It is rather nauseating the way both parties routinely and hypocritically switch sides about whether obstructing nominees is good or evil, every time the Presidency changes sides. (At least the current hijinks aren't blatantly against the Senate's written rules, the way the nuclear option was!)
In the long run, it's best to allow the President to fill any vacancies which arise during his term with reasonable candidates. The alternative equilibrium, in which a party division between the Senate and Presidency leads to no appointment, will just make life harder for everyone in the long run. The Republicans can't seriously believe that the Democrats won't retaliate once they retake control of the Senate.
Also, this seems like very bad timing for Senate Republicans to take this particular stand, seeing as the two people most likely to be President next care even less about the Constitution than the current one. St. Hillary Clinton seems to believe that Congressional acts can and should change the meaning of the Constitution. And I don't think Donald Trump (who would discriminate against Muslims, and already has abused the power of eminent domain) has any more genuine concern for the Constitution than he does for the teaching of Jesus! (Maybe Republican pressure would keep him on the straight and narrow when it comes to appointments, but then again maybe not.) Most likely it will be Clinton, in which case the Republicans will have eroded faith in the process without actually getting anything to show for it.
(Unless their previous posturing has caused St. Obama to nominate somebody more moderate than he otherwise would have, which actually seems fairly likely now that I think of it. But this is only relevant if the Senate actually confirms Garland!)
Waiting to confirm based on who the next President is, also seems like a unwholesome habit for the Senate to acquire. An 8 member court is not the end of the world, but 4-4 tie votes are a bit annoying. Also, since the next Supreme Court term begins in October, delaying the confirmation until after the elections in November is awkward, since then Garland (if confirmed) would have to rehear any important cases.
So I think that the Senate should confirm Garland, who seems to be an all around decent person unlikely to try to shift court precedent extraordinarily far to the left. At least, no more than one would expect from Scalia being replaced by a moderate. (In fact I am a little concerned he believes in judicial restraint so much that he won't protect civil liberties quite as much as the current liberals do.) But it is hard to determine his true opinions based on his current record as an appellate judge, bound to follow Supreme Court precedent. In the event that he is appointed, we shall just have to see.