It would be an act of insane stupidity to give johnson this mandate and unifying him with the brexit party vote, the worst tactical move and that's without considering that preventing a no deal brexit so the bannon tribe can get paid is absolutely the right priority for anyone with any morals whatsoever
TBH this notion that the Tories will win because they'll promise Brexit might well be one of those political orthodoxies that everyone believes but is actually wrong.
It would be almost embarassingly easy for any opposition to point out, entirely truthfully, that the Tories generally and Johnson in particular were responsible for delivering the Brexit they promised and have failed to do so. Johnson voted against a deal twice, Rees-Mogg twice, Patel and Raab three times - they caused the first delay (from March 2019) themselves. The 2017 election was fought on a promise to deliver Brexit - "Brexit means Brexit", don't forget - and they failed to do it; Johnson negotiated his latest deal and failed to deliver it because of his actions in losing the support of the DUP and his own ex-Tories.
In short if voters want Brexit to be delivered, the Tories are the people who failed to deliver it up to now. I'd expect Labour at least (probably the BP as well) to be hammering home that message and I'd be amazed if it didn't work.