CINCINNATI—Here in Hamilton County, where one in seven people lives beneath the poverty line and budget cuts have left gaps in the schools and sheriffs department, residents are bracing for more belt-tightening: rollback of a property-tax break promised as part of a 1996 plan to entice voters to pay for two new stadiums.

The tax hit is just the latest in a string of unforeseen consequences from what has turned into one of the worst professional sports deals ever struck by a local government—soaking up unprecedented tax dollars...