No it isn’t.
When, as a very young fellow, I became a Democrat party worker, I was told most definitely by the party “warhorses” that it’s “Democrat Party”, not “Democratic Party”. It had to do with the profound belief that the party is a “party of Democrats”, which, at the time, was thought to denote a group of independent thinkers who nevertheless agreed on fundamental issues, rather than a group that would determine a “party line”, willy-nilly, by “democratic vote”. That might have been a localism, but if it was, it was pretty widespread.
I used the term “Democrat Party” then, and I have continued to do so, notwithstanding that it has fallen out of fashion and notwithstanding that I can no longer support the party in which I was a worker and an officeholder. I use the term, not in derision, but in honor of the old line party leaders, mostly now dead, who taught me “how to say it right”. Those folks were my political mentors and worthy of my respect, even now.
If that offends you, I regret that it does. But I’m not going to change the way I express it. I trust you are a sufficiently understanding person to realize that not all nomenclature ought to be viewed as offensive simply because some take it so.