In his book The Power Elite, Mills argues that capitalism, at least as practiced in post-WWII America, leads to something of a subversion of democracy. Capitalism leads to the existence of a power elite that controls all of the main aspects of the country.
Mills argued that there were three major centers of power in the United States, all of which were populated by an interlocking group of people with close ties to one another. These three centers of power were politics, business, and the military. Capitalism gave the business class the ability to be part of this power elite. Capitalism allows businesses to become very large and very powerful. When this happens, the leaders of the business community come to have a very strong influence over the government. This is particularly true, Mills says, because the business leaders come from the same social circles as the political leaders and have a great deal of influence with them.
Thus, to Mills, capitalism helps to bring about a situation in which democracy is weakened. Capitalism leads to the existence of a power elite that reduces the level of democracy in the United States.