In one word. Demographics. There has been infiltration of the system for a long time, most notably the Frankfurt School academics and others like them who were so influential in fermenting the spread of Marxism through the institutions. But the ultimate thing that will bring about communism in America, and indeed much of the western world is demographics. If you look at the voting intentions of many of the new voting blocks who have been imported into the west, they often tend to vote for bigger government. Once the demographic rubicon is crossed in America for example, the US will be a left-wing one party system.