gosub
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this is the centre piece of her administration. this is the culminating moment of brexit. the moment that the agreement she has negotiated is put to the vote. everyone else has said they're fine with it, this is when we find whether the uk's up for it. nothing else theresa may does as prime minister compares in importance to whether this part of her work is crowned with success - or failure. so obvs she has to go if there's a heavy defeat, she's nailed her name to this particular mast.
Not only that but it would be impossible for her to tear her deal up and negotiate something different that might get through Parliament