Because of the two party system, you have to either vote Democrat or Republican for your vote to count, so your actually leftist ideals, which are to the left of the centrist Democrat party, do not meaningfully exist as a voting block.
So the only political movements that you think are allowed to be said to exist in a country are those explicitly and broadly represented by a major party?
I mean, I guess that's a position you could take, but it seems like a pretty extreme one.
