The Constitution mandates that the President shall nominate a candidate to fill a vacancy on the Supreme Court. It does not say may nominate a candidate. No such mandate exists to confirm a nomination. Both Democratic and Republican controlled Senates have refused to confirm nominations offered by sitting Presidents.
For the very first time, however, to my knowledge, a majority leader of the Senate has actually asked a President not to nominate anyone, giving rise to speculation that, said leader, wishes to avoid the political embarrassment of the opposition citing his own words, about the necessity of giving a Supreme Court nominee an up or down vote, or to refer to a 7 year record of his party obstructing a President from the very day of his first inauguration.
I heard, this day, a Republican spokesman plead that Obama should delay the nomination in order to bring the parties together, as if, at the 11th hour, they, the Republicans, just might give up partisan politics, in return. Of course, after he does so, it's more likely that they will impeach him for not meeting his obligation under the Constitution.
In any event, it should be appreciated if my Republican friends would cease sending me articles dealing with the blocking of confirmations, by Democrats, as they are not germane.