William F. Buckley: https://www.youtube.com/watch?v=lTyZAul60ok

Mr. Burns: https://www.youtube.com/watch?v=othBTFk_W1Y

There's an ambiguity with "mid-Atlantic".

Sometimes it means "trans-Atlantic", an accent which Buckley, Burns, and many pre-1950 movie stars spoke with (while on-screen anyway). It falls somewhere between the accents found in the USA and accents found in the UK.

In the context of this thread, I think it means the mid-Atlantic states in the US. It's close to the American "television news accent".

EDITED TO ADD: It's worth noting that the two accents called "mid-Atlantic" sound very different.

That was the intent, yes. But I was neither born nor raised in the mid-Atlantic states, and people are generally surprised when I tell them I was born in the deep South because I don't have the expected southern drawl. "Mid-Atlantic" these days is usually a stand-in for "non-distinct American accent".

I really like this (no longer updated) website devoted to north american accents https://www.aschmann.net/AmEng/

Old Hollywood movies: https://en.m.wikipedia.org/wiki/Mid-Atlantic_accent

That said, people today often use "mid Atlantic accent" to mean the standard "region-less" accent used by, for example, TV announcers. It is more and more the standard accent of America's educated elite in urban centers, regardless of location.

Generally any everyday local news host has a variation of this kind of accent. There are different regional accents around the country, but generally news show hosts tend to have a plain sounding generic American sound that may not match up with the local drawl. For example, Alabama/Georgia populations have a particular southern accent but news out of Mobile and Atlanta doesn't sound local. Similar for Boston or NYC, etc.

It's the accent you hear on TV when you think the speaker has no accent.

https://en.wikipedia.org/wiki/Mid-Atlantic_accent