> The Democratic and Republican parties have structurally built themselves into the election system in a way that shuts out any attempt to gain traction by third and fourth parties.
It's more than that: The Democratic - (and arguably the Republican party at one time) - are functionally "big tent" parties. For example, the Democratic party has a progressive faction, an "establishment" faction, and a centrist/moderate faction.
The republican party has a big tent too. There are neocons, extreme far right people, religious people, disaffected workers, anti globalists, and increasingly Hispanics and other immigrant groups.
These groups often have little in common. Just look at trumps support base.
It's more than that: The Democratic - (and arguably the Republican party at one time) - are functionally "big tent" parties. For example, the Democratic party has a progressive faction, an "establishment" faction, and a centrist/moderate faction.