There we go, acting like a leftist because someone disagreed with you.

Mostly state level where you need to have a certain number of signatures in support or votes from last election to be on the ballot. Legislatures are majority/minority with no power given to independents though they are still classified as caucusing with one of the two parties. Lastly, many federal agencies (like the FEC) are codified in law as having to be split between Republicans and Democrats with the chairman being the deciding vote and of the President's party. Oh, and the EC is winner take all in 48 states and DC, which also shuts out a 3rd party run as you have to win outright in a majority of states without the existing party structure to get out the vote, to raise money, and to beat back frivolous challenges to your campaign.