Fully agree, but posted this more as theory on a potential way out of a two party hegemony. It also requires a lot of time and trust building to have any effect, so probably not applicable to US's current crisis. But there are a lot of countries that still use FPTP in some fashion, it might be applicable there.

For the average voter it's not much different than a regular political party: vote for this candidate if we endorse them. The actual math and projections of when to trigger this can be coordinated and agreed on by people you/your local representative trust.

The theory is simple but it probably would be trial and error in practice. For example, if our actual turnout votes for an endorsed candidate differ from our pledged voters by X% then we need to increase our number of pact members proportionally before triggering.

Very similar, but that's only at the national level and controlled by establishment parties. I could see, for example, Texas + Florida + other red states falsifying their election results or ramping up legal disenfranchisement to capture blue state votes.

There's also no inherent political unity in that agreement, any state could (should?) break the pact if they think it's in the best interest of their voters. Either way I think they would be careful not to allow anything that significantly disrupts the system.

This theoretical system is driven by popular policy, no popular vote or fair representation comes into play. There could be a far right psuedo-party in the same way as a progressive left. At the end of the day achieving a critical mass is still needed to trigger the pact/win the election.

In practice it might not get as far to the left/right as you want, but this release valve would prevent the Overton window from being pushed way outside of popular opinion. I think a key outcome would be pushing in election reform candidates, eventually replacing this impromptu system with proper representation.