Perhaps the number of competitors has become too large. The best-selling hybrid in the world, both currently and in history, suffered a sales drop of 14% in August. The once invincible Toyota Motor Corp. (NYSE: TM) Prius has struggled recently.

The troubles the Prius suffers from now extend across the entire year so far. August sales dropped 14.3% to 23,474. Sales through the first eight months dropped to 11.5% to 151,213. Despite the falloff, it still ranks among the top 20 selling cars in the United States.

The Prius’s track record over time is extraordinary. Launched in 1997, the Prius has sold nearly 4 million units worldwide. It has gone through four entire redesigns since then, and currently offers a plug-in model. It has at least tried to cash in on the movement to electric cars.

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During the past several years, many of the best-selling cars in America (along with a number that do not sell so well) have launched hybrid editions. Even Toyota has hybrid versions of its best-selling models, particularly its top-selling Camry. In other words, in the hybrid sector, Toyota competes with itself. The decision does make sense based on wide consumer preferences for size, configuration and price. However, it cannot help but cannibalize Prius sales.

The wild success of the Prius has also driven the world’s other major car companies to press aggressively into the sector. For example, Ford Motor Co. (NYSE: F) offers hybrid or electric versions of its Fusion, Focus and C-Max. The Fusion and Focus are among the top 20 selling cars in America this year. Honda Motor Co. Ltd. (NYSE: HMC) offers hybrid or electric versions of its Accord, Civic and CR-Z. All three are among the top 20 selling cars this year — another indication of how crowded the hybrid market has become.

The Prius’s major problem appears to be that it has become a victim of its own multiyear success.

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