Wall yourself through the process of creating this third party. Let's say its this leading edge european-style-leftist party that cares about people, for real this time.

This party starts growing slowly, but where do the people come from? Maybe some people who avoided politics altogether until this new party came along, but most people will likely come from the Democratic party.

So the party is building and the democrat party is shrinking, while the republican party stays the same. They may even see some growth from the "fracturing" of the Democrat party. They start winning more and more elections as instead of a race coming out 46% to 44%, it comes out 46% to 36% to 10%.

That ratio keeps building in favor of the new leftist party, but we lose seats and elections every cycle. And then eventually (maybe stupidly hopeful?) The new left party completely takes over the democrat party which ceases to exist.

Now we are back at a two party system, but have lost the country for maybe 5, 10, 20+ years? You could argue this is a better idea than what we currently do, which is try to change the party you are part of slowly over time with voting and campaigning, but I personally wouldnt say that myself.