In total, Maryland has added 24 new taxes or fees in recent years, Change Maryland says. Florida, which has no income-tax, has been a large recipient of Maryland's exiled wealthy.
“Maryland has reached the point of diminishing returns. We're taxing people too much and people are voting with their feet," said Change Maryland Chairman Larry Hogan. “Until we change our focus from tax increases to increasing the tax base, more people are simply going to leave, leading to a downward spiral of raising revenues on fewer citizens."
The finding adds to the renewed debate over raising taxes on the wealthy. In New Jersey, Gov. Chris Christie recently vetoed a millionaire's tax passed by his legislature, while California and other state governments are also considering higher taxes on high earners to fix budget problems. President Obama on Monday asked Congress to extend tax cuts for those making $250,000 or less – effectively increasing taxes for the higher earners.
Many contend that higher taxes drive out the highly mobile rich, who can simply move to a lower-tax state or even lower-tax country. Recent data shows that a record 1,800 Americans renounces their citizenship last year.
Some argue, however, that there is little real evidence that higher state taxes drive out large numbers of high earners. Neil Bergsman, director of the Maryland Budget and Tax Policy Institute, said while a number of people left the state between 2007 and 2010, others moved in. The net loss, he said, is “very small,” he said.
What’s more, he points out that the wealthy usually move because of a job change, life change or retirement rather than taxes.
“There is no evidence that tax structures are a significant determinant in their location choices,” Bergsman said.
What’s more, he said, Maryland is still minting high-earners and has among the highest incomes and highest concentration of millionaires in the country.
Other studies in New Jersey, Connecticut and Rhode Island have also failed to offer proof that taxes are the main driver of out-migration by the top earners. (See here and here). In some states, weather is a bigger driver of out-migration by the wealthy than taxes.
Still, with top earners paying the largest share of taxes in many high-income states, many politicians don't want to take the risk of raising tax rates further.
An earlier version of this story incorrectly said President Obama would ask Congress Monday to extend tax cuts for those making $250,000 or more. In fact, it was for those making $250,000 or less.
-By CNBC's Robert Frank
Follow Robert Frank on Twitter: @robtfrank