The ad was a response to Sen. Scott Brown's claims — in an effort to raise money — that Maddow is planning a run against him in 2012.
"Hi, I'm Rachel Maddow. I host a TV show on MSNBC. I also live in Western Massachusetts, in the beautiful hilltowns of Hampshire County," the ad begins.
"I'm not running against Scott Brown," Maddow said. "I never said I was running against Scott Brown. The Massachusetts Democratic Party never asked me to run against Scott Brown. It's just not true. Honestly. I swear. No, really."
Maddow hit Brown for trashing one of his constituents.
"Do you remember when Mitt Romney ran for President after being our Governor and he went around the country insulting Massachusetts, talking about what an awful state we are?" she wrote. "To have our new Senator raising money around the country by saying how terrible one of his Massachusetts constituents is, kind of feels the same way to me.
Maddow wrote that "it's standard now for conservatives to invent scary fake threats to run against," citing death panels and the birther movement. "Senator Scott Brown's only been in DC seven weeks, but he already seems to be fitting right in with how conservatives operate there."
Maddow said she didn't run the ad because she's running against Brown, "but because he's the Senator for all of us, and maybe this will make him think twice the next time he wants to smear one of his constituents to raise money out-of-state."
The ad concludes with a reminder that Brown has refused to appear on Maddow's show.
"My show airs at 9PM Eastern in Massachusetts on MSNBC," she wrote. "So far, Scott Brown refuses to come on. Maybe he'll change his mind -- I hope he does.
See the ad below: