The TPM story suggests that the Angle campaign was simply complaining about Reid's use of its copyrighted material. If that were so, I would think the copyright claim would be weak; the fair use and First Amendment arguments in favor of allowing a political candidate to post his opponents' campaign material in order to comment on it would be strong. See, e.g., Keep Thomson Governor Committee v. Citizens for Gallen Committee, 457 F. Supp. 957 (D.N.H. 1978) (rejecting copyright claim in campaign context; emphasizing that "The First Amendment affords the broadest protection to such political expression in order to assure the unfettered interchange of ideas for the bringing about of political and social changes desired by the people.").
But it's clear from the C&D letter that the Angle campaign is not actually making a copyright claim. Rather, they are concerned that the Reid campaign, by posting the former Angle web site, which included a form for supporters to submit their names and email addresses, may have been harvesting those on their own, and misleading Angle supporters who genuinely wanted to submit their names to her campaign -- not Reid's. Indeed, the actual demand in the C&D letter is confined to this point:
[W]e demand that the Reid campaign immediately cease its nefarious actions of requesting and obtaining the names, email addresses, and zip codes under false pretenses from unsuspecting supporters of his opponent, Sharron Angle.According to TPM, the Reid campaign acquiesced and took down the site. It's one thing to post an opponent's words to comment on or criticize them; I would support the Reid campaign's right to do just that. But it's quite another to trick supporters of one candidate into giving their contact info to the other, and it appears Angle had every right to complain about that aspect of Reid's tactics.
Update: Several people have pointed out to me that the C&D letter includes some broad language about the Reid campaign's use of copyrighted material. That's true, but I think that in context, what Angle is really complaining about is the use of that copyrighted material to mislead her supporters into submitting their contact info to the wrong site.