Democracy in America

This is the only reference I can find for the word "mores" in the text. Is this the passage and central idea that you're questioning.

I have previously remarked that the manners of the people may be considered as one of the general causes to which the maintenance of a democratic republic in the United States is attributable. I here used the word manners with the meaning which the ancients attached to the word mores, for I apply it not only to manners in their proper sense of what constitutes the character of social intercourse, but I extend it to the various notions and opinions current among men, and to the mass of those ideas which constitute their character of mind. I comprise, therefore, under this term the whole moral and intellectual condition of a people. My intention is not to draw a picture of American manners, but simply to point out such features of them as are favorable to the maintenance of political institutions.

The mores (pronounced more-Ays) are not a group of people. Mores are the essential, characteristic customs and conventions of a community i.e the habits, traditions, and practices of a certain people. Think of it in terms of the way of life is lived within society.