Mr. Prechter, 61, lives in Gainesville, Ga., where he runs Elliott Wave International, a forecasting and publishing firm. He graduated from Yale as a psychology major in 1971, dabbled as a singer, drummer and songwriter in a rock band and became a technical analyst for Merrill Lynch.

He became fascinated by Mr. Elliott’s writings, which suggest that the market moves in predictable if complex patterns. Along with A. J. Frost, Mr. Prechter wrote “Elliott Wave Principle,” a 1978 book that predicted the emergence of a great bull market  a forecast that was largely fulfilled. By 1987, he was widely regarded as an expert in technical analysis. Articles in The New York Times said he was known as “the market’s leading technical guru”  and more. An article in October that year said he had “emerged as both prophet and deity, an adviser whose advice reaches so many investors that he tends to pull the market the way he has predicted it will move.”

He has far less day-to-day influence now, after years spent developing a theory he calls “socionomics,” which holds “social moods” as the cause not only of market cycles but also of economic and political events. A grand cycle is ending, he says, and the time for reckoning is near.

In 2002, he published “Conquer the Crash,” which predicted misery ahead. Even so, he said in 2008 that the market would soon rally sharply  then said late last year that stocks were about to fall and that the great decline would resume.

Since 1980, the advice in his investing newsletters, when converted into a portfolio, has slightly underperformed the overall stock market but has been much less risky, losing money in only one calendar year, according to calculations by The Hulbert Financial Digest. Mr. Prechter said he disagreed with the methodology used in these measurements, but offered none of his own.

For his part, Mr. Acampora says that the Elliott Wave has some validity as an indicator but that “it’s only part of the story” of technical market analysis, which also needs to be buttressed by economic and fundamental research.

Mr. Prechter says his unifying theory, socionomics, is a “young science.”

“We’re quantifying it,” he said. “We’re working on it.” In the meantime, he contends, it has enabled him to “look around the corner” and prepare for a dangerous future.