As individuals, they are literally colleagues. As legislators, the Ds regularly vote with Republicans to make all sorts of things happen. Corporate profits are highest under Democrats. The military budget expands under both parties. Democrat presidents regularly continue the policies of their Republican predecessors, the most obvious being Obama/Biden+Clinton and their war on Libya. Ds and Rs celebrate together, dancing in the halls of government. They eat together, send their kids to the same schools, are members of the same social clubs, have far and away vastly overlapping interests in insider trading, property values, procurement contracts, global domination, suppression of domestic revolt, etc.

It would be obvious that they collaborate if it wasn't for the fact that PR professionals out number journalists 6-to-1 in the USA. The parties are PR firms, psychologically manipulating the masses into complacency, directing their energy towards dead ends, and keeping everyone fighting each other by turning everything into a moral issue that can only be solved by voting more morally.