The Obama campaign's top lawyer fired off a letter to Karl Rove Thursday, demanding a retraction of a "mystifying" comment Rove made and raising questions about his upcoming appearance at a Mitt Romney campaign event.

The letter is the second that Bob Bauer has sent to Rove this week. The first argued that Rove could no longer insist that his advocacy group, Crossroads GPS, was policy oriented -- a distinction that allowed it to shield the names of its donors. The follow-up letter, obtained by The Huffington Post, makes that same point, arguing that there is no "social welfare" component to the group's operations.

But it also challenges Rove in more direct terms. Bauer hints that Rove, the chief strategist to former President George W. Bush, is colluding with Romney, the presumptive GOP presidential nominee, by homing in on Rove's presence at a Romney retreat in Utah this upcoming weekend. He also expands the scope of his complaint to Rove's role with American Crossroads, the super PAC arm of Crossroads GPS.