But both parties have leavers & remainers as supporting voters.
Maintaining the current status quo ("our Brexit is better than their Brexit") will lead to a similar GE outcome
Labour will be no more attractive to Tory voters than they were last year and the same goes for Tory party
a) convince enough leavers that Brexit is bad and they will soften or halt it.
b) convince enough remainers that #Brexit is good and that they are the best party to make it successful.
This is the route
The point is however the tory vote is majority leave and Labour is majority remain/soft Brexit.
Labour will struggle to convince enough of its remainers that leaving is a good idea