These strike me as the most plausible optimistic and pessimistic scenarios for the top three candidates:
TED CRUZ UPSIDE SCENARIO: A COMMANDING POSITION BY FEBRUARY 20
Cruz wins Iowa going away. This isn’t hard to imagine; in fact, I’d guess today that he wins by 8–10 points there. With some momentum into New Hampshire, Cruz finishes second there, behind Trump. For good measure, Rubio is beaten by, say, Chris Christie, and finishes fourth. The “establishment lane” fails to produce a strong, viable contender and Cruz goes mano a mano with Trump in South Carolina and gets enough mainstream Republican support to beat him and establish a dominant position for the nomination.
TED CRUZ DOWNSIDE SCENARIO: TRUMPED!
In what would play as an upset, Trump beats Cruz in Iowa. Cruz has no momentum in New Hampshire and finishes fourth or fifth there. He goes into South Carolina trying to stop Trump, who has probably gone two for two in Iowa and New Hampshire. Even worse would be if Rubio finished strongly in both Iowa and New Hampshire and is competing with Cruz for very-conservative and somewhat-conservative voters. Cruz loses South Carolina and begins to look like a Trump spoiler going into the rest of the South.
TRUMP UPSIDE SCENARIO: BRING YOUR BROOM, BECAUSE THIS THING IS A SWEEP
Trump wins Iowa. Maybe it’s a surge of new voters. Maybe he’s gotten more serious about attacking Cruz and it has worked. Maybe he pulls something like his Muslim-travel-ban proposal in the last few days and it pushes the race his way. Validated by his win and with momentum, Trump posts a crushing win in New Hampshire, with more mainstream Republicans still fractured four ways. Better, John Kasich finishes a distant second there. The establishment is panicked but with Cruz weakened and no other viable candidate emerging and the even more massive media coverage from his victories, Trump wins South Carolina. He has now shifted to a more serious, unifying tone, befitting his dominant position in the nomination battle.
TRUMP DOWNSIDE SCENARIO: HOWARD DEAN, CALL YOUR OFFICE
Trump begins to look a little different to voters as the caucuses near. His supporters who aren’t committed Republicans don’t turn out. He underperforms his polls and loses badly to Ted Cruz in Iowa and slips into third to Marco Rubio. Experiencing a major downdraft and not reacting well, he slips in New Hampshire and loses, say, to Rubio or Christie. Trump goes into South Carolina a two-time loser and finishes third behind Cruz and Rubio or Christie. After all the media attention and the bravado, Trump is on a path to nowhere.
MARCO RUBIO UPSIDE SCENARIO: PEAKING AT JUST THE RIGHT TIME