You vote Democrat. You don't vote Democratic. You are therefore a Democrat voter.
You honestly can't see how that is plainly incorrect?
(I often wonder how many people who use this phrase incorrectly are deliberately trolling, or have simply been misled by those who are.)
I'm pretty sure you've had this exact same argument with Carl before. It's obvious that the people who do it only do it because it pisses Democrats off.
The "people who did it" here was a Buffalo's TV station website. It's not uncommon to see newspapers or news websites use Democrat as an adjective, particularly when referring to voters, not the Democratic Party itself. This is especially true in headlines, or in other cases where space is at a premium.
Democrats have no inherent right to tell the rest of us what words we can use.
Fine, you people who vote Republic are welcome to use whichever words you want.
