A democracy is essentially about determining the course of our nation together. To do that, it helps a lot to have a good citizenry. A good citizenry is informed, serious about things that are worth taking seriously, and not liable to be led off course by demagogues. (Everyone doesn't have to be like this, but you need a critical mass of people who are.) But I've always thought that a good citizenry is also composed of people who assume, until proven wrong, that many of the people who disagree with them are acting in good faith.
This matters for policy: you're unlikely to choose sound policies if you assume that anyone who disagrees with you is a depraved, corrupt imbecile. It's hard to learn anything from people you have completely written off. But it's also corrosive to any kind of community or dialogue to assume the worst about large numbers of people you've never met. It makes you less willing to try to take their problems seriously, and to try to figure out how they might be solved, or to try to understand what's driving them.
Montesquieu taught us (Spirit of the Laws, Book III, section III) that democracy can only survive by the cultivation of virtue: as he put it, virtue is a necessary spring of democratic government. Law and power are not enough: you need character. Without it the whole community falls apart:
When virtue is banished, ambition invades the minds of those who are disposed to receive it, and avarice possesses the whole community. The objects of their desires are changed; what they were fond of before has become indifferent; they were free while under the restraint of laws, but they would fain now be free to act against law; and as each citizen is like a slave who has run away from his master, that which was a maxim of equity he calls rigour; that which was a rule of action he styles constraint; and to precaution he gives the name of fear.
And without this absolute prerequisite, the people blur into a mob, and mobs call forth demagogues to manipulate them, and demagogues rise to be tyrants and abusers of power. Americans will recognize this idea from Washington's Farewell Address, which uses Montesquieu's claim as the basis for an exhortation to the American people to create institutions for the diffusion of knowledge, to exercise responsibility with regard to public debt, and to show impartial good will to other nations. The Address also warns against the Spirit of Party, and partisan spirit is one of the great enemies of that virtue that is the cornerstone of democratic government.
The virtue underlying such government does not consist of every virtue; it is a civic virtue and therefore is devoted to very particular ends. The basic conclusions Hamilton (who wrote the first draft of the speech) and Washington drew from it are pretty much exactly right, although they focused less on the domestic front than the foreign front: it consists in continual self-education, vigilance with regard to one's civic responsibilities, and a basic respect and good will for one's fellow citizens and fellow human beings simply because they are your fellow citizens and simply because they are fellow human beings. Such respect is no doubt very limited; but, even so, without it government by and for the people ceases to be by and for the people. It is that respect and good will that prevents the government from being an instrument of partisan grasp for power. Parties are perhaps inevitable in any sort of democratic government: people will disagree. But if a democratic republic is to survive, the good of the people must come first and the good of one's side must be firmly subordinated to it. Without an elementary respect and good will for one's fellow citizens, this cannot happen.