[Blueflag]

It's simple really. He is fundamentally incoherent.

in·co·her·ent/ˌinkōˈhi(ə)rənt/

Adjective:
1. (of spoken or written language) Expressed in an incomprehensible or confusing way; unclear.
2. (of a person) Unable to speak intelligibly.

Synonyms: disjointed - rambling - desultory - disconnected

[Blueflag]

He comes across as incoherent because he is dissembling.

dissembling = present participle of dis·sem·ble (Verb)

Verb:
Conceal one's true motives, feelings, or beliefs.
Disguise or conceal (a feeling or intention).

... and he's just not that good at it when he doesn't have his teleprompter to read from.

[chessplayer]

Adjective:
1. (of spoken or written language) Expressed in an incomprehensible or confusing way; unclear.

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Libs call it “nuance.”