A complaint alleging that Sen. Thad Cochran (R-MS) was living in a illegal apartment in Washington DC was filed by Phil Kerpen, an influential conservative activist in the efforts to overturn Obamacare.
In a since deleted tweet, Kerpen asked the Washington DC Department of Consumer and Regulatory Affairs (DCRA) “@dcra Is there a legal basement rental unit at 218 Maryland Ave NE?.” According to Matt Orlins of DCRA who provided a screen shot of the tweet, “Generally, it is not our policy to release the names of complainants. In this case, the original complaint came through via Twitter, which is public.”

Cochran is currently in a very close primary against Tea Party hopeful State Sen. Chris McDaniel. The election, which is next week, has gotten increasingly dirty as both sides have exchanged a number of negative ads. It crescendoed when Clayton Kelly, a pro-McDaniel blogger, broke into the nursing home where Cochran's bedridden wife lives and took pictures of her. He has since been arrested along with several co-conspirators, including a McDaniel volunteer and a former radio co-host of McDaniel. The McDaniel campaign has strongly denied any link to the break-in or to Kelly.
The complaint, which was first reported by Matt Boyle of Breitbart News claims that Cochran aide Kay Webber, who rents a basement apartment in her Capitol Hill row house to the Senator. The allegation is that Webber has never registered her home with city government to be registered for commercial purposes. If guilty, it’s a civil violation that would be punished with a fine of up to $2000.
Kerpen currently runs a group called American Commitment. Prior to that, he spent years as a vice president at the influential conservative non-profit Americans for Prosperity. On Twitter, Kerpen made clear that this was a "[s]imple inquiry, not a complaint." He went on to say that his "tweet was deleted because @DCRA said on they prefer not to discuss investigations on Twitter."
In an email exchange with The Daily Beast, Kerpen said he was simply curious. "I figured why not simply ask and find out. I thought it would be a simple yes or no question whether the required permits are on file - if I got a reply at all," he said. "It did not occur to me that it would result in an investigation." Kerpen went on to note that he has no involvement in the primary. This is simply a case where curiosity didn't kill the cat, it just meant the conservative activist inadvertantly filed a complaint with DC city government.