I suggest you read Ludwig von Mises' "Anti-Capitalist Mentality". It is a very good explanation about the thought process that gets professionals - engineers, doctors, intellectuals, college professors, journalists - to embrace socialism. I believe a lot of these people fit in that income bracket.
More than that, it would be the younger individuals in those professions, I would guess.
My assumption was that 50-75 represented most salaried and blue collar workers as they have gained seniority (and, obviously, age).
75-100 is mainly those who are starting out in the "professional" category.
100-150 would be professionals with more age and small business owners.
150-200 would be more business owners and those who deal with the rules the government has created.
200K+ are those who have "made it" and now feel guilty.