According to many linguists interviewed by Vox in the article linked to below, Jackie Kennedy’s unique-sounding speech patterns are an example of several different dialects. For the new movie Jackie, Natalie Portman studied her accent by listening to her interviews on YouTube over and over.
Linguists say that some of her linguistic features, like dropping her R’s at the end of syllables, reflects both her New York regional background and her background as an affluent person from the American Northeast born in the 1930s. R-dropping reflects the style of many upperclass American communities, which sought to emulate some of the features of the British accent, which was viewed as more sophisticated. However, Kennedy interestingly presented an even more “extreme” r-dropping than many New Yorkers, dropping it even when it forms the stressed syllable, like in the first syllable of the word “furniture.”
Like all politicians, and indeed everyone else, Jackie also changed her style of speech based on her audience. Because Portman used filmed TV interviews, quite a specific context, it may not be the best representation of how she would have spoken in other diverse settings. On the whole, linguists seemed to agree that Natalie Portman’s accent in Jackie was fairly believable.