One of Justice's Scalia's bedrock principles was following his interpretation of the original intent of the Constitution. Conservatives who follow this idea call themselves strict constructionists and frequently accuse liberal judge's of "legislating from the bench". Of course, this is all nonsense, because the framers of the Constitution would have nothing to say about abortion, or net neutrality, or even civil rights (considering that slavery was in the Constitution).
But, okay, you're a conservative and you're for following the strict text of the Constitution. Here's what the Constitution says about putting people on the Supreme Court:
[The President] shall nominate, and, by and with the Advice and Consent of the Senate, shall appoint Ambassadors, other public Ministers and Consuls, Judges of the supreme Court, and all other Officers of the United States, whose Appointments are not herein otherwise provided for, and which shall be established by Law:
The President shall nominate.
It doesn't say, The President shall nominate unless he's in the last year of his term.
It doesn't say, The President shall nominate unless Congress is controlled by the other party.
If you claim to be all high and mighty about following the Constitution, then you cannot hold the position that the Senate should filibuster or ignore the President's nominee. You can't have it both ways.
And for those Republicans who are saying that we should wait until after the election "for the voters to decide", they already did. The voters elected President Obama to be the person to nominate Supreme Court Justices until he leaves office in 2017. That's reality. That's the Constitution.
Suck it up, put on your big boy pants and do your damn job.