Here's an OP in GDP by madfloridian that is just

[Jim Lane]

26. I understand these issues and I think this case isn't as clear as you imply.

First, I agree with your general point that the right-wingers frequently misuse "Democrat" where "Democratic" would be correct, that they do so deliberately for its political effect, and that it should be called out.

As to my level of understanding, I've helped get the correct information out there by editing the Wikkipedia article on the subject. (For those who don't want to bother clicking through: There's a separate article on the Democratic Party. I've linked to the article that's specifically about the "Democrat Party" solecism.)

Now, as to this instance: English, unlike some other languages, allows a construction in which two or more nouns are rammed together. It has a technical name that I forget. As an example, the body that governs world chess is known even in English as FIDE, from its French name, Fédération Internationale des Échecs. The English translation is World Chess Federation. In French, however, you can't just put three nouns in a row like that, so it's noun-adjective-prepositional-phrase. Similarly, DU is a message board or an Internet forum.

Turning (finally) to McCaskill, she is, individually, a Democrat, just as she is a woman. You could call her a female Senator (adjective-noun) but you could also call her a woman Senator or a Missouri Senator (each a noun-noun construction). By analogy, I think "Democrat Senator" is acceptable, even though "Democrat Party" or "Democrat caucus" or the like is not.

ETA: After posting this I finally noticed that randys1 had been blocked and couldn't respond. For the record, I don't agree with randys1 on the substance, and I agree that his/her tone was unnecessarily combative, but I don't agree with the block, either.