I am not American but I know one party represents the right the other represents the left. But, both parties have shown to be pragmatic and sometimes contradictory in their positions:

i.e. pro-life Democrats, pro-Obamacare Republicans, etc...

• What are the key differences they will never agree and give identity to each party?
• Is there any kind of difference that will cause the expulsion of a member of the Republicans or Democrats for holding view too close to the opposition?