Neither of these candidates wants to hear this right now, but if Donald Trump wins South Carolina by double digits, then South Carolina has to be the end of the line for whichever one of them finishes behind the other. There's a decent chance that Trump may have mortally wounded himself with his 9/11 truther antics over the weekend, so maybe he finishes second place, or pulls off a narrow win. In which case, maybe Jeb or Rubio can afford to continue jockeying for position behind Trump and Cruz.
However, we have all thought on numerous occasions that Trump was due for a reckoning, and it's never happened. If Trump's supporters haven't left him for mocking a disabled guy, making fun of McCain's military service, or calling Ted Cruz a "p*ssy," then you have to assume they won't leave him for being a 9/11 truther, either.
I've written here before that the assumption that Trump has hit his ceiling at 35% is completely unfounded and ignores historical trends of GOP voters. And besides, even if Trump stays at 35%, that's enough for him to dominate the delegate count if the field does not thin behind him sooner rather than later.
John Kasich is dedicated to his insane strategy of waiting until after Super Tuesday to seriously contest a state. John Kasich is not interested in listening to reason or to the will of conservative voters, so even though he's already screwed, there's no real point in telling him that on the front page of RedState. He's mainly fighting for independents and Democrats anyway.
Rubio and Jeb, on the other hand, are fighting for the exact same pile of Republican voters, and the time has come for them to settle this issue decisively. By the time the dust clears, one way or another, from South Carolina (or at the very least after Nevada), one of them needs to realize the jig is up and go back home, for the good of the party.
Realistically, Rubio is in much better shape than Jeb nationally, and has performed better overall in Iowa (where he trounced Bush) and New Hampshire (where he narrowly lost after Bush spent $40M). But if Bush is able to beat him in South Carolina (and especially Nevada) he needs to understand that his path to victory is essentially blocked, his nomination is stalled, and come back and try again next time, for the sake of not dooming this party to crushing electoral defeat at the hands of Trump.
I realize that both men had high hopes entering this campaign, and that both have developed a personal dislike for one another. But after South Carolina - and at least after Nevada - one or the other needs to do the right thing for the country and pack it up.