To start us off, most people in the USA don't seem to understand the relationship between our voting system and our political parties.

As this video demonstrates, a First Past the Post voting system will always eventually result in two dominant parties, just as a reality of optimal political strategy.

No conspiracy or effort is needed by those in power to keep two parties in place, though that's not the same as saying they don't each still takes steps to ensure they are the most powerful parties.

Even if a third party arises and gets enough votes to pose a challenge to the others, it will do so by leaching so many votes from whatever party most resembles it until the opposition of both begins winning every election, necessitating a shift to the new party by all those remaining.

For example, let's say a truly Libertarian party arises in the US: its main distinguishing features is that it's completely against the drug war, social programs, and all foreign aid.

The only way it would arise in power is by largely cannibalizing the Republican party, since it shares their economic values, if not their social ones. There might be some overlap with Democrat voters, but not nearly enough to balance both.

So the only remaining Republicans would be the extreme social conservatives, who would have to either be unrepresented as they watch the Democrats win election after election, or choose to jump ship to the Libertarian party. Alternatively, there might even be a substantial group that would throw its weight behind the Democrats, if they care enough about things like foreign aid (conservative Jewish coalitions) or keeping drugs illegal (assuming the Democrats remain against legalization).

TLDR: We will always have two dominant parties in this country as long as our voting system encourages one vote and winner-take-all. If you want to change that, don't advocate for voting third party: advocate for changing our voting system.