"In going to Sicily you are leaving many enemies behind you, and you apparently want to make new ones there and have them also on your hands. Even if we did conquer the Sicilians, there are so many of them and they live so far off that it would be very difficult to govern them. It is senseless to go against people who, even if conquered, could not be controlled, while failure would leave us much worse off than we were before we made the attempt. The right thing is that we should spend our new gains at home and on ourselves instead of on those exiles who are begging for assistance and whose interest it is to tell lies and make us believe them, who have nothing to contribute themselves except speeches, who leave all the danger to others and, if they are successful, will not be properly grateful, while if they fall in any way they will involve their friends in their own ruin."
It probably wouldn't make any difference. Thucycides reports that "the few who actually were opposed to the expidition were afraid of being thought unpatriotic if they voted against it, and therefore kept quiet."