Trump Wasn’t Always So Linguistically Challenged. What Could Explain The Change?

It was the kind of utterance that makes professional transcribers question their career choice:
“ ... there is no collusion between certainly myself and my campaign, but I can always speak for myself — and the Russians, zero.”
When President Trump offered that response to a question at a press conference last week, it was the latest example of his tortured syntax, mid-thought changes of subject, and apparent trouble formulating complete sentences, let alone a coherent paragraph