Do you think the democrats will legitimately provide a challenge to FPTP voting? To the electoral college?
In time, with enough constituents demanding it, yes. The electoral college specifically hurts their chances.
Do you think the republicans will?
They would have no power without the electoral college. They are close to losing even with a disproportionate amount of influence which is why they are willing to sabotage the country to get back into power.
How would that get overturned, what would that actually look like?
The best way to challenge this captured system is to recapture it via one of the two parties. Make sure your representatives know what you want, convince others who agree with you to do the same, and convince the rest that disagree with you that you are in the right. Then you get critical mass and make it happen. The same way every cultural change happens.
Alternatively or simultaneously, you could have efforts pushing third parties locally. It is easy for an established party to destroy one political opponent on the national level. Just throw money at the problem. 1,000 local elections though? Much harder to manage an attack on a thousand fronts.
