The way I’ve been categorizing it has been whether the person is trying to encourage others to abstain from voting or voting for a third party.

Sounds like you’re only categorizing it as propaganda if they explicitly call for someone to not vote. “Both sides suck” is essentially the same thing, just without the explicit call to inaction. Not to mention the secondary benefit of painting over the GOPs crimes by making it sound like the dems are just as bad.