The Toyota Prius, once revered as the greenest car on the road, has fallen on hard times. Sales are on a six-year losing streak, and now the previously pre-eminent eco-mobile has fallen behind the Ford Fusion hybrid — a model its parent company plans to pull the plug on in a couple years.

"The Prius is the model that got us to where we are today; it led the charge to electrification, but now it's facing so much competition," said Jeff Schuster, senior vice president of forecasting for researcher LMC Automotive. "The Fusion is having a little bit of a last hurrah to send it off on a higher note."

Monthly sales released earlier this month provided further evidence of Prius's slide.