Tocqueville was one of the most important political philosophers to discuss the idea of democracy, particularly when it came to "modern" republics. Many of his ideas guide the ways in which we think about democracy today.
One of Tocqueville's ideas was the belief that a commitment to the common good was essential to democracy. Tocqueville thought in this way because he understood that democracy is a type of majority rule. This means that any action of the government will be guided by the opinions of the majority. The majority, however, is prone to mistakes and division. If people are not committed to the common good, this might lead them to vote in a way that benefits them only, or that brings some advantage to the group they belong to. This would lead to injustice and inequality, and would result in a democracy that does not work well.