Illinois Governor Pat Quinn is the latest Democrat to demand a tax increase, this week proposing to raise the state's top marginal individual income tax rate to 4% from 3%. He'd better hope this works out better than it has for Maryland.

We reported in May that after passing a millionaire surtax nearly one-third of Maryland's millionaires had gone missing, thus contributing to a decline in state revenues. The politicians in Annapolis had said they'd collect $106 million by raising its income tax rate on millionaire households to 6.25% from 4.75%. In cities like Baltimore and Bethesda, which apply add-on income taxes, the top tax rate with the surcharge now reaches as high as 9.3%—fifth highest in the nation. Liberals said this was based on incomplete data and that rich Marylanders hadn't fled the state.

Well, the state comptroller's office now has the final tax return data for 2008, the first year that the higher tax rates applied. The number of millionaire tax returns fell sharply to 5,529 from 7,898 in 2007, a 30% tumble. The taxes paid by rich filers fell by 22%, and instead of their payments increasing by $106 million, they fell by some $257 million.

Yes, a big part of that decline results from the recession that eroded incomes, especially from capital gains. But there is also little doubt that some rich people moved out or filed their taxes in other states with lower burdens. One-in-eight millionaires who filed a Maryland tax return in 2007 filed no return in 2008. Some died, but the others presumably changed their state of residence. (Hint to the class warfare crowd: A lot of rich people have two homes.)

A Bank of America Merrill Lynch analysis of federal tax return data on people who migrated from one state to another found that Maryland lost $1 billion of its net tax base in 2008 by residents moving to other states. That's income that's now being taxed and is financing services in Virginia, South Carolina and elsewhere.