Yes, I have noticed that for at least 40 years... There's more to it too: Most Pubs are simply not as adept in debates, interviews, and such, as Dems. Why? Because very often the Dem came up as a lawyer and / or with a Poli-Sci background. The Pub politician is more likely to be a business person, Dr., etc., lacking that background, and is also LESS likely to be the sort of person who during at least a portion of their lives spent extensive time in the bars around low-info voters... Part of President Trump's "ability" lies in him just naturally being exceptionally intelligent and fast on his feet, combined with the time he spent in show biz, his experiences in construction (I'll bet he can run a backhoe with the best of 'em), and so on.)
Further, we can complain about the MSM taking advantage every time a Pub says something stupid, and giving Dems and Socialists a pass, but, hell, it's not like our candidates don't know that: Reasonably politically-wise Pub candidates guard their comments very carefully unless (like Trump) they are WELL experienced into turning virtually any seeming disadvantage into advantage, or, rendering it unimportant.
Take Mitch McConnell for example. He's not very well liked, even in KY, but he's politically adept, runs vicious attack ads against opponents (and very "sweet" ads for himself), always manages to be the better of poor choices in the general election, debates reasonably well, and very importantly, makes few big campaign blunders.
I have thought for some time that the Republican Party needs to offer some sort of minimum 2 year training for candidates: Not so much on issues, but on how to present themselves and avoid major campaign blunders, while at the same time not everyone looking and sounding like they came out of a mold...
I think you’ve covered it quite well and accounted for the differences in political performance in the majority of cases. This instance isn’t one of them but sometimes their actions or statements are so dumb that background and training shouldn’t even be a factor.