Reagan Brothers Clash: Anger Over Alzheimer's Allegations A memoir by a former president's son has touched off a family quarrel of words.

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.

Ron Reagan said he was troubled by one of the president's 1984 debate performances and was concerned enough to spend a day shadowing his dad at work.

"It wasn't like oh my God he doesn't remember he's president...You know, it was just -- I had an inkling something was going on," he said.

Ron also writes in his book: "I've seen no evidence that my father (or anyone else) was aware of his medical condition while he was in office. Had the diagnosis been made in, say, 1987, would he have stepped down? I believe he would have."

The former president was diagnosed with Alzheimer's six years after leaving office. He died on June 5, 2004.

He wrote an eloquent letter to the nation, saying: "I only wish there was some way I could spare Nancy from this painful experience..."

His wife Nancy Reagan, said goodbye to him seven years ago.

"We've learned of the terrible pain and loneliness that must be endured as each day brings another reminder of this very long goodbye," Nancy Reagan said in 2005.

ABC News' Leezel Tanglao contributed to this report.