The federal alternative minimum tax (AMT) was enacted in 1969 after Congress learned that 155 affluent Americans had actually paid no federal income taxes. Wisconsin passed its own version of the AMT in the 1980s.

The measure is designed to capture at least some revenue from individuals who have so many deductions or credits they otherwise would owe no income tax at all.

“Maybe the easiest way to explain it is that the AMT makes sure that generally higher income filers can’t claim significant deductions to where they whittle their tax liability down to nothing,” says Dale Knapp, research director of the Wisconsin Taxpayers Alliance.

Republicans have had their sights on the alternative minimum tax in the past. Rep. Dale Kooyenga (R-Brookfield) had proposed eliminating it as part of a package of reforms he proposed last year.

Conservatives have also called for eliminating the AMT, saying it acts as a second income tax and complicates an already complex system.