The constitution is clear on a nomination to the Supreme Court. The President nominates and the Senate advises and consents. The Senate can advise the President that his choice is unacceptable. Under the rules of the Senate, the Senate leader can make this call.
That is all there is to it. No need to nuance it with the “Biden rule” or anything else. The constitution is all the matters.
Obama is a lame duck who has already done lasting damage to the separation of powers that undergirds our constitutional system. He has already put his stamp on the federal judiciary: Besides two Supreme Court justices, he will have placed well over 300 like-minded, life-tenured appointees on the bench by the time he leaves office. He should not be permitted to further shape the ideological direction of the Supreme Court, especially with several cases on the horizon that challenge Obama policies implemented by unilateral, legally dubious executive action.