There is no room for a third Party in a FPTP voting system as per Duverger’s Law. I think it’s likely the party would never take off, no matter how sensible its positions are. And even in the best case of it slowly growing while eating away at the democrats to eventually have a majority: during the time it takes to get there, elections would just be an easy republican win every time, as the ‘left half’ is split between bernie and status quo. I don’t think that’s worth it.