Reagan Brothers Clash: Anger Over Alzheimer's Allegations

Jan. 17, 2011— -- A memoir by a former president's son has touched off a family quarrel of words.

In an exclusive interview with ABC News, President Ronald Reagan's son Ron Reagan said he saw early signs of Alzheimer's disease while his father was still in office.

Ron Reagan recounts the signs in his new book, "My Father at 100: A Memoir."

His half-brother, conservative commentator Michael Reagan, has fired back on the allegations, calling Ron an "embarrassment" to his parents.

In a series of tweets over the weekend, Michael Reagan accused Ron of trying to "sell out his father to sell books."

Michael Reagan added, "my brother was an embarrassment to his father when he was alive and today he became an embarrassment to his mother."

Early Signs of Alzheimer's Disease?

"There was just something that was off. I couldn't quite put my finger on it," Ron Reagan told "20/20's" Elizabeth Vargas.

Ron Reagan said it bothered him when he saw his father making phone calls and reading note cards like "he had prompts."

"These seemed like conversations that wouldn't really require that," he said.