It all depends on Ben Carson!
It is mid December. Trump, defying all predictions is still towering over all his republican rivals, capturing as much as a third of the primary voters. In a crowded field, while establishment republicans – who remain a minority of republican primary voters - have resigned themselves to rallying around Rubio,
Ted Cruz appears to be emerging as Trump’s primary challenger. Ted Cruz’s fate will be decided by how well Ben Carson performs in the early primaries, particularly in Iowa (Feb 1) and in South Carolina (Feb 20).
If Carson performs poorly in Iowa and South Carolina, then Cruz will have enough of the evangelical voter base supporting him to perform well on Super Tuesday. On the other hand, if Carson does well (comes among the top 3 either in Iowa, or in South Carolina), there would be little incentive for Carson to drop out, thereby cutting into the evangelical vote that could otherwise go to Ted Cruz.
If the evangelical vote base continues to stay divided, Trump would handily beat the establishment candidate (Rubio), and Cruz. Wthout the undivided support of the evangelical vote, Cruz does not appear to have the voter base to defeat Trump.
The longer the republican primary remains a four way race, the easier it is for Trump to gain momentum, and the key to keeping it longer as a four way race depends on how the voters of Iowa and South Carolina feel about Carson.
Enough uncertainty to vex the republican establishment.