If he comes out against Brexit now, that would help kill TM's deal and maybe Brexit itself, but it snatches away that juicy GE carrot that's dangling just out of reach.
Once the GE is called, he can be as Remainy as he likes, if that's what
On a Remain platform, combined with a hopeful positive plan for change, Labour would win.
Campaigning for Brexit for the second GE in a row, they'll lose and potentially even come 3rd.
Like trying to thread a needle blindfolded... he *might* hit the eye, but he's much more likely to miss the needle or impale himself on it.
They could go into a GE with a fully spelled out Brexit position (but still seek Brexit) AND an explicit commitment to rescind Article 50 and remain if their particular flavour of Brexit proves unattainable.