What is the largest barrier of all the barriers that hinder 3rd party candidates in the US?

The largest barrier they face is funding. The elephants and the donkeys have their hands in special interest groups pockets. They are the life blood behind the elections. The more money you have to promote your campaign, the better chance youhave to be elected.

Sad, but a very true fact

Barriers to raising money are only one thing. The main barrier is the media and the League of Women voters, and other pro-politics groups. They are prejudiced against anything but the two-party rule. They have the final say as to which, if any, minority party gets access to TV and Radio, which is the best way to gain exposure. They also control access to public forum debates, and if I recall correctly, Ralph Nader was forcibly removed from a scheduled debate with the two major party candidates in the last presidential election. Whether or not you support such a candidate, their actions were inexcusable!

The biggest barrier is peoples attitude. Every time someone says your going to throw away your vote. If people had the fortitude of there convictions they would vote there minds. But since no one seems to nothing will change in Washington. Other countries have up to 20 parties and seem to get along fine. Republicans and Democrats have to much money and power to let that slip away.

Democrat and Republicans. They are not going to share the power so they create new laws to make it difficult to 3rd party candidates win. Also by stating that they will not let a 3rd candidate do any thing if elected..

Confidence of block voters (voters who vote for one of the two major parties because they won't actually look at the issues. They trust party lines to support the issues that are important to them. )

Legitimacy

Two words: Name Recognition.

It's a tie between "voter laziness" and "the two-party oligarchy."