"Who's on the case when it comes to the flat tax?"

In this quote, Joshua Micah Marshall is questioning who is taking charge and examining the idea of a flat tax. A flat tax is a tax system where everyone pays the same percentage of their income, regardless of their earnings level. Marshall is questioning who is actively looking into this concept and its potential ramifications. He may be suggesting that there is a lack of attention or action being taken towards this subject. By asking this concern, Marshall is highlighting the significance of understanding and analyzing the prospective results of a flat tax system.