The only two restraints—at least for now—on public union governance in Illinois are GOP Gov. Bruce Rauner and the state’s flat income tax. Democrats hope to do away with both in November.

Democrats in Springfield have filed three constitutional amendments to establish a graduated income tax—the rates won’t be determined until after voters give their assent. The state’s flat 4.95% income tax, believe it or not, is lower than the rate in all its neighbors save Indiana (3.23%). But its property and corporate taxes are among...