I'm not 100% on that, but I'm pretty sure that when the reset button is pushed in January, 2017, any nominee who was rejected or otherwise not approved can be resubmitted.
It permits the President to play an extremely not-lame duck role in this election. The President can pick an extremely qualified candidate, let the Republicans refuse to hold hearings, mention the refusal at every press conference, and begin to make a couple dozen Senatorial elections turn on the idea that no matter who's running for President, it's also a mandate for President Obama's Supreme Court nominee.
So, to pick a hypothetical example which is totally not going to happen because we are all adults, if one of the Democratic candidates somehow doesn't get the nomination and the other candidate is unpalateable to a particular voter and that voter unwisely decides not to participate in the Presidential election, that voter may still have an interest in unseating his or her Senator, see?
And that puts that hypothetical person in the booth, and unless that hypothetical person is insane, that person will think really damned hard about voting against Donald Trump.... And that person can tell all of DU that he or she was only there to cut the rope on that person's Senator. And nobody will ever know! Hypothetically, that is.
I just know someone is going to take it personally, but I swear I'm not trying to single any one of you out in the hypothetical example above. Oh no, I am singling all of you out, hahahaha!