You can read Limbaugh’s reasoning here, courtesy of RushLimbaugh.com:
Rush: I’ll tell you what ought to be happening here. IRS agents should be interrogating Harry Reid to determine who it is that supposedly leaked confidential tax information about a private citizen to him. And Harry Reid, having disclosed publicly what he was told, should be investigated as well for passing such information on to the public. This is a felony, what Harry Reid is doing. So if anybody needs to be investigated here, it’s Harry Reid. IRS agents ought to be talking to him and trying to figure out who leaked. You know, who is this guy from Bain that’s calling Harry Reid? Who are these people? And then he publicly disclosed what he was told, and he doesn’t know if it’s true. He’s demanding that Romney prove it, so Harry Reid’s admitting he doesn’t know if it’s true.