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.

"I understand that you will [be] traveling to join Governor Romney this weekend at a luxury fundraising event in Utah –- an appearance that press reports note 'could raise questions because of campaign finance laws barring any coordination between super PACS and actual campaigns,'" Bauer wrote, likely alluding to American Crossroads, the super PAC arm of Crossroads GPS. "But I would ask that this trip not delay the immediate correction of your misstatements I am requesting."

(READ THE FULL LETTER BELOW)

The letter also picks up on an exchange between the two that was escalated Wednesday night when Rove accused Bauer of having a political vendetta against him that was rooted in earlier legal conflict.

In an appearance on Fox News, Rove insisted that Bauer had represented Dana Jill Simpson, the former Republican operative in Alabama who served as a whistleblower against Rove and others in the case of former Gov. Don Siegelman.

"This is the kind of guy Bob Bauer is," Rove told Fox News host Greta Van Susteren. "I know what he’s doing. It is not going to change us in one way shape or form from doing exactly what we’re entitled to do under the law."

But it turns out that Bauer never represented Simpson. Nor, he wrote in the Thursday letter, did he have "anything at all to do with her allegations about you or any appearance by her on 60 Minutes."

"The identity of her lawyer is a matter of public record – a lawyer I have also never met," Bauer added. "Please promptly correct these remarks on the record."

Bauer's letters allowed him to make his point without actually putting many legal or political resources behind the effort. As Rove argued during the Fox News interview, there are also progressive 501(c)4 institutions that, like Crossroads GPS, toe the line between policy work and campaigning, and Bauer hasn't asked for the names of their contributors.

But none of those groups do it with as much gusto or money as Crossroads GPS does. A top Obama campaign official on Wednesday floated the idea of going to the courts in order to push for donor transparency, suggesting that the campaign is getting more and more serious about the matter and may soon move beyond sending letters.

A request for comment from Crossroads GPS was not immediately returned.

(READ THE FULL LETTER BELOW)

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