We have a first past the post single member district electoral system.
What that means is that all you have to do to win is get the most votes ~ not even a majority ~ in a district.
The consequences for party politics are profound. The math necessary for winning tends to reduce the number of potentially successful competitors to one or two parties. Coalitions are necessary since, in general, in our society, there's almost never a single voting block that constitutes a majority.
NOTE: (although courts have tried to do that with black voters ~ creating one black Congresscritter and effectively removing the black vote from surrounding districts ~ as if having a black Congresscritter offsets total loss of political influence outside the black milieu).
We end up with districts of all sort dominated by one or the other of the two parties ~ and sometimes dominated by only one (such as is the case in New York districts).
Mathematically, the ideological structure of American electoral politics is called a BI MODAL SADDLE.
So-called "independents" and "moderates" have no clear choice except this ~ if they want their votes to count they have to vote for someone ~ and that someone is going to come from one of the "modes" on that "saddle".
At the same time any so-called "independent" or "moderate" is going to find himself working OUTSIDE the parties without a label. That's probably why you never find an "Independent party boss" (bwahahahahah!)
Sometimes in relatively evenly balanced districts where control passes from one mode to the other on a regular basis it is possible for sufficiently motivated outsiders in the fringe of the bi-modal saddle to toss an election one way or the other. Still, the person they are voting for will arrive in the state legislature, city council or Congress prepared to caucus with one of the two modes in the bi-modal saddle as it is represented at the top of the heap.
So, you ask, what about the RINOs ~ where do they come from?
A RINO (and there are Democrat equivalents) is usually an individual whose ideological inclinations are with the regular Democrats but he or she doesn't fit for social, racial, religious or other reason. Their odds of ever acquiring sufficient backing from their fellow Democrats to run for public office is limited. If the local Democrat "machine" is sufficiently powerful, this sort of potential office holder has few options in our system. He can't just go form a new political party. So, he ends up running as a Republican in a district (or state) with a weak Republican presence.
This person's election as a Republican may give the Republicans CONTROL of a legislative house, or a Congressional house, or the city council. Once control is achieved, the most numerous party then sets the legislative agenda!
NOTE: (Newt Gingrich is in the school of thought that it's more important to get control than to have candidates who adhere to Republican principles, and he was successful at gaining control of the House, but not the Senate. There were entirely too many RINOs in the Senate to ever achieve a stable situation for the Republicans. Hence the Jeffords disaster which set this nation back 150 years).
Rahm Emanuel is a practitioner of the art of gaining control of Congress. He is the fellow who decided the Democrat litmus test should eliminate GUN CONTROL. The Democrats were then able to run pro-gun candidates in weak Republican districts (or where we had RINOs) and take enough seats to control the agenda in the House. That's why we have that horrid Nancy Pelosi in charge).
The Old South sought to eliminate blacks from the electoral process while at the same time maintaining some semblance of democratic process. That's why you had extensive primaries, and then primary run-offs in their old system. Today, with few Democrats left in the South outside of all-black judicially mandated districts, you farely see the run-offs happening but I don't think the laws were changed ~ just the opportunities to run for office.
You can pick up all this stuff from V.O.Key's books regarding parties and politics. It's old stuff ~ some of it nearly 80 years old ~ but he was on top of the situation even then.
Now, back to the "independents" and "moderates" ~ a "moderate" is in the eye of the beholder. For instance, I see no "moderate" Democrat voters. They are all ideologically driven fanatics. And the "independents are usually just Democrats who find it hard to make decisions ~ their bosses should keep close watch over them.
Cheers!