Using the term 'Rat on occasion is a shorthand way to flag particularly corrupt, illegal or disingenuous behavior by Democrats, let off steam and have a little fun. You have to admit that the 'Rats' behavior often differs by truly unconscionable degrees from what they profess so that a somewhat jarring term is suitable, IMO, when discussing these instances, since similar pejoratives would usually be employed to describe equivalent behavior absent partisan considerations. Also, reference to their partisan doings as 'Democrat' rather than 'Democratic' which is a political process serves to differentiate the party from what it often has no association with. Furthermore, it is merely a contraction of 'Democrat', so it is arguably not name calling;)
It's usually preferable to behave conventionally, with the maximum level of civility, but sometimes refusing to deviate from norms allows one to be taken to the cleaners, particularly in politics when dealing with corrupt hypocrites and demagogues such as the 'Rats are. OTOH, as soon as Lefties completely disavow their partisan rhetoric, which they are giving no signs at all of doing,of course, it would probably be best for conservatives to also drop the ad hominems.
LOL, I like your explanation in post 64 the best.