The flaws of FPTP voting are generally well known at this point. Extremely popular policies are given no platform in the US two party system. But could a grassroots network of vote compacts negate the spoiler effect?

A big-tent psuedo-party could hold a parallel primary before elections, agreeing to use all votes for a candidate if a critical threshold is reached. A green light candidate would need 51% (+ X% margin) of internal votes and ~40% of total election votes (varying by historical election turnout). Otherwise the voters default to least evil of the two party system.

The first question is legality, which I have no clue on. However, political parties are built on the idea of shared voting power, so I don't see how any argument against this scheme would make sense.

The second question would be logistics. Validating public voter identities is easy enough, but there would need to be a system of representative conventions to maintain trust. A local group proving unity by winning a local election would grant them access to a higher tier, up to the national level.

Obviously there are more complexities in reality (eg: the US electoral college, real life voter loyalty, etc...), but could it work?