Smaller parties... like the SNP? 7% of the seats from 4% of the vote. Seems to fly in the face of what everyone says about what is possible under FPTP. And what's more they've been effective in influencing what happens in parliament!
Here's the "law": https://en.wikipedia.org/wiki/Duverger%27s_law
So you might think it's possible. But it's a case of in theory, in theory and in practice are the same, in practice, they ain't.
And that is what FPTP is designed for. To over-reward those at the top of the leaderboard. It's designed to not be representative.
It's actually not that different in practice There are a large range on views in both parties, but you put on the Democratic or Republican label in order to build a majority coalition. This is sort of like how the Westminster parliamentary system works: labor forms a government with the greens. Labor has more seats by far is and is mostly in charge, but the greens get some government positions and a little influence on the agenda of state.
As a recent example of this here Bernie Sanders was not at all a Democratic party member but "joined" with them to try and get enough support.
They got 7.4% of the votes at the last election and 1.9% of the seats. Smaller parties like the green party do even worse.
By design.