1. Either by exposing the ridiculous pointlessness of a Johnson soft Brexit
Or
2. By exposing the vicious destructive cost of a Johnson hard Brexit
Either way means next leader can only be a remainer.
From the responses its clear people have not thought this thru.
An opposition whether they have a policy to rejoin or not (likely not) simply cannot politically critique Johmson's Brexit from a Brexit POV.
It wont work. It wouldn't be credible.
It would pre-program defeat.
Only way a pro Brexit Labour leader makes any sense is if you believe that not only will Brexit be reasonably good for UK, but also beneficial for working class Labour supporters.
Otherwise you'd repeat same mistake that got us into this mess.
You cant out Brexit the Tories.