'Go with your members and get booted out of any sensible position' I don't understand why its to be taken as a given that noone will vote for the anti-brexit pa

'Go with your members and get booted out of any sensible position' I don't understand why its to be taken as a given that noone will vote for the anti-brexit party, given that a was a good 45% of the population supported it? surely then all you have to do is win 10% of the population over with other arguments to win?