Perhaps!
In speech, one cannot tell the difference between a
democratic platform and a
Democratic platform, a
democratic poster and a
Democratic poster.
Democrat solves this problem and what one uses in speech falls naturally into what one writes, especially at the casual level of a forum.
Words don't get their parts of speech designation from some committee, which, after much study, finds that there is a gap in language that needs filling.
There are not many ways for new words to enter the language; by far the most common one is from the people who use language. They invent new words daily and Democrat (adj) could simply be another new word.
Thomas, did you read my new posting in the thread,
Should there be a gender-neutral form of he or she pronouns?
http://www.able2know.org/forums/viewtopic.php?t=68416&start=40
It points up that you can use the grammar portion of your Chicago Manual of Style as firestarter without fretting that you've lost anything.
Imagine, a university publication that has been around that long, and, which has acquired some measure of respectability for defining citation use, and they ask Bryan Garner to write the grammar portion.