None of the individual Republican factions represents a majority of the party. In order to get the nomination, therefore, one must find a way to obtain support from at least one other factions.
The most plausible groupings are the Romney (Christian Democrat and PBP) and Reagan (PBP and Reactionary) coalitions. A CD and Reactionary coalition, based on an agreement on social issues, opposition to illegal immigration, and economic policies targeted to assist the white working class, is a logical possibility, but has little or no precedent, and is unlikely.
The Romney coalition (generally viewed as the Republican establishment) would presumably take the following positions:
a. Tepid opposition to abortion and same-sex marriage;
b. Support for a generous immigration policy;
c. Tax cuts targeted to assist both the rich and the working poor;
d. A more aggressive foreign policy;
e. Some measure of business deregulation; and
f. Some watering down of the ACA, but not a full-blown repeal.
The Rubio tax cut plan is an excellent example of an attempt to build a bridge between the CDs and the PBPs. The initial version of the plan apparently emphasized tax cuts for the working poor, a CD idea, but that approach was insufficient to obtain support from the PBPs, so the final version included sweeping changes to benefit the extremely wealthy.
The Reagan coalition (named after a man who, we sometimes forget, was viewed as a dangerous insurgent by elements of the GOP prior to the 1980 general election) would take the following positions:
a. Vocal opposition to abortion and same-sex marriage;
b. Strong opposition to any attempt to liberalize immigration policies;
c. A larger defense budget, but a reluctance to intervene militarily except under extreme circumstances;
d. Benefit cuts for the (undeserving) poor;
e. Financial and environmental deregulation;
f. Across-the-board tax cuts primarily benefiting the wealthy; and
g. Total repeal of the ACA, and its replacement by. . .nothing.
Bush, Christie, and Rubio will be attempting to win the nomination by initially becoming the standard bearer for the Romney coalition. Similarly, Trump, Cruz, Walker, and the rest will have to prevail in the Reagan coalition primary before taking on the Romneys. The nominee will probably be the person who wins his subprimary first, and who is then able to run again a divided opposition coalition, as Romney did in 2012. Wise candidates will consequently focus their firepower on opponents within their coalition in the early stages of the campaign.