Guessing the Nevada and South Carolina Results
Iowa and New Hampshire might well be the high water mark for Sanders. There is no indication of a surge for Sanders in South Carolina. In the national polling data, two of five recent polls have Sanders either close or leading while three of five have Clinton with a modest but solid lead. Thus, he would probably need a surprise upset in Nevada to have a chance at changing the narrative. The other game changer would be a new scandal, more smoke/fire from existing ones or some health issue.
Update: Barring new scandal, more smoke/fire from existing ones or some health issue, it is over. So far, there is no indication Sanders is moving the needle in South Carolina. He will run to the end to try to influence the party but that is all she wrote.
As for the Republicans, they have a primary in South Carolina. According to the polls, Trump will probably hold on for the win for the same reason as in New Hampshire - his opponents dividing up the rest of the voters.
The open questions:
Will the endorsement of Rubio by Gov. Haley carry Rubio to a number two finish edging out Cruz at the lean to the tape?
Will Kasich and/or Bush do well enough to stay in the race?
My guess is that Rubio will finish number two over Cruz fairly easily.
If either Kasich or Bush or both finish under 10% the calls for them to drop out will become very loud and at least one will heed those calls. Unfortunately, I think they both will cross the 10% line allowing them to continue.
Update: Bush is out. There is little data on the Nevada caucus but until the race comes down to just two or three candidates, Trump will continue to run the table with wins (~ 30% of the vote) as the rest of the candidates divide the votes. Kasich and Carson claim they will continue but there will be mounting pressure for them to get out so that it is a fight between Trump/Cruz/Rubio. Even then it isn't clear a 3-way race could break Trump's hold.