This report is a few months old, but particularly timely.
The Paul Simon Public Policy Institute at Southern Illinois University conducted a poll last fall and asked Illinois respondents what revenue-generators they would support. The most popular way for the State to raise more money was a federal-style progressive income tax where higher incomes pay a higher rate than lower incomes. 65.9 percent of respondents favored a progressive income tax.
Interestingly, while 74.2 percent of respondents who make less than $50K annually favored a progressive tax, and 68.9 percent of those who make between $50K and $100K favored the tax, more than half of the people who make more than $100K and would presumably pay the higher rate still favored a higher rate for themselves. 57.5% of the respondents who make more than $100,000 support a higher rate for higher incomes. (See page 26 of Professor Charles Leonard's Public Policy Institute Occasional Paper #12 to read the report yourself.)
Governor Quinn's proposal to raise the state's flat rate income tax rate from 3% to 4.5% combined with the uber-progressive move to triple the amount of tax-free income from $2,000 to $6,000 is as close to setting up a progressive tax as the General Assembly can get, since the Constitution unfortunately mandates a flat rate.
So while it is not only the right thing to do to tax lower income people less than higher income people (why would we possibly tax people who are earning money below the poverty line?), according to the SIU poll, it is also the most popular option on the table -- even among the people who would pay more.
I suspect a progressive income tax is popular because it is the right thing to do.