I am of the opinion that the term "politically incorrect" is almost synonymous with "offensive to the left". I believe that it is overwhelmingly liberals defining what is too offensive in any given context.

I offend conservatives regularly, so from my point of view, political correctness is not exclusive to the left. It's more a matter of how the paradigm shifts. Sometimes the pendulum just swings too far and needs to come back to a reasonable middle. I think that's the gist of Seinfeld's opinion.

I like to use the word "retarded" as an example of this phenomenon. 'Retard' means "to make slow" and 'retarded' meant 'a slow learner'. So now a newer PC term is 'developmentally delayed', which is quite literally a synonym for 'retarded'. How did this happen? When I was a kid, if our friends were doing something weird or illogical, we'd call them retarded. Basically, we added a negative connotation to the term when applying it to people of average or above average intelligence. So, a group of people decided to defend the dignity of the retarded by using an alternative term. That just resulted in the next generation of kids using the term 'DD' to insult their friends! Now some people use the term 'cognitively impaired' or something similar. Ultimately, it doesn't matter what terminology is used if it's still referring to the same thing. I swear that they just keep using longer terms, not because they're more descriptive, because it's harder to use them as an insult! That's the ultimate point, of course. We shouldn't be using these terms as insults. Whatever the term may be, our mothers' old saying still holds true: If you can't say something nice, don't say anything at all.

Given Seinfeld's (an admitted Democrat) and opinion toward this kind of attitude, is there a chance this could be splintering or damaging the Left?

No. SJW's are all about defending dignity... sometimes with really stupid reasoning behind it. Their place is still on the left with me, even if I don't think their reasoning has any substantial affect on defending the dignity of others.