South Carolina

Some things to note regarding the South Carolina Republican Primary:

(1) South Carolina (SC) is probably the most important primary for Republicans, because
(a) it is held in a typical Republican stronghold (religious-conservative Southern state in the "bible belt")
(b) four of the last five primary winners in SC went on to win their party's nomination
(c) a candidate's chances will diminish close to zero if he is not able to score at least one Top3-finish in the first three contests (up to South Carolina), which means SC is probably the very last chance for Bush and Carson.

(2) The remaining Republican candidates may be attributed to approximately the following three-and-a-half GOP wings:
(a) NPO - "Nationalist Populist Outsider" wing (nationalistic/populist): Trump
(b) RTT - "Reactionary Theocratic TeaParty" wing (ultra conservative/theocratic): Cruz (Southern Baptist), Carson (Adventist)
(c) CWE - "Conservative War-Machine Establishment" wing (neo-conservative/militaristic): Rubio (very conservative), Bush (conservative)
(d) MCE - "Moderate Conservative Establishment" wing (moderate-conservative wing of CWE-wing): Kasich

(3) Potential of each GOP wing in the SC primary:
a) NPO wing (Trump): slightly above 30%
b) RTT wing (Cruz, Carons): approx. 30%
c) CWE wing (Rubio, Bush): slightly below 30%
d) MCE wing (Kasich): below 10%

(4) Expectations for SC and prospects for the following contests:

a) Trump is favoured to win. With a clear win in SC he would prove that he can also win in Republican strongholds. For Trump to - surprisingly - miss the first place would be a severe blow for him, whereas a decisive win with a margin of >10% would further bolster his momentum.

b) For Cruz SC should be a good territory. But Carson's candidacy will cost him a few points within the RTT wing, therefore a Cruz win in SC would be a big surprise. To keep his chances alive he will definitely have to win some (Southern) states in the coming weeks. In SC he should at least pull off a strong second place, to keep Rubio at distance, (what he probably will achieve). A third place finish would be a severe blow to his aspirations.
For Carson SC is probably the very last chance: If he can't score here, he won't succeed anywhere.

c) Rubio urgently needs a good result again. To miss the Top3 again could be fatal for him. If he were to finish at second place, his chances could improve rapidly to establish himself as the only serious Republican still standing not named Trump. In that case his chances as "lowest common denominator" of the CWE wing could keep him alive all the way until the convention.
For Bush South Carolina is probably the last chance: If he misses the Top 3 again by far, he probably won't win anywhere. He then might suspend his campaign quite soon. On the other hand his millions of cash-on-hand could keep him alive at least his home state primary in Florida - and hope for a miracle on the Ides of March.

d) Kasich will be chanceless in South Carolina (as well as probably in most other Southern states). His (small) path to the nomination relies on hoping for the Northern and Western states, and for the minimal chance that most of his opponents could self-destruct faster than he would be forced out of the race.