Massachusetts lawmakers are expected to debate a proposed constitutional amendment calling for what's been dubbed the "millionaire tax."

The House and Senate have scheduled a constitutional convention Wednesday to consider the 4 percent surtax.

It would apply to individuals with annual incomes above $1 million. The higher tax rate would only apply to that portion of income over $1 million.

The current income tax rate is 5.1 percent.

Supporters say it would raise money for education and transportation. Critics argue it would hurt the state's business climate.

The measure must receive the backing of at least 25 percent of lawmakers meeting in the joint constitutional convention during two successive legislative sessions to secure a spot on the ballot.

The earliest the measure could go before voters is 2018.