A comment at Square State in early January claimed "[t]he problem with US politics is that there are three major constituencies (christian nationalists, business, and labor) but only two parties." Predicably, I think that the coalitions behind the two major political parties in the United States are quite a bit more complicated. But, what are those coalitions?
For the Democrats, there are probably at least four major components: (1) labor (including non-union economic populists), (2) social liberals (including big business professionals, trial lawyers, professors, teachers, liberal clergy, government employees, the urban middle class and the secular left), (3) ethnic minorities, and (4) residual Dixiecrats (particularly long serving, pre-realignment elected office holders).
For the Republicans, the components include: (1) Conservative Christians and xenophobes, (2) big business senior management and the rich, (3) small business owners and farmers, and (4) neo-conservatives.
Of course, components of each coalition have overlap with each other. This is why they form coalitions with each other in the party that they are in, rather than the other party. There are notable exceptions: law enforcement unions often lean GOP as do minority members who are Cuban; organic farmers who run small businesses and Northeastern suburbanites lean towards the Democrats.
Conservative Christians and labor bring the ability to mobilize warm bodies willing to work to their respective parties. Big business and social liberals, respectively, bring money to their respective parties. Ethnic minorities and small businesses bring loyalty and commitment to their issues. Residual Dixiecrats and neo-conservatives bring a willingness to exercise political power.
This model of two political parties with four parallel factions each is also oversimplified, but it better explains intra-party politics than a three faction, two party model.