Nissan Leaf and Chevy Volt

Carlos, we have a problem.

U.S. sales of the Chevrolet Volt extended-range plug-in bested the Nissan Leaf battery-electric's for the fifth straight month, with Leaf sales falling behind last year's pace for the first time in 2012, giving pause for those subscribing to Nissan chief Carlos Ghosn's vision of steadily increasing EV sales.

General Motors' Chevy division sold 1,760 Volts in June, more than triple the 535 Leafs sold last month. June represented the second-best month ever for the Volt, which moved a monthly record 2,289 in March. For the year, the Volt has sold 8,817 units, meaning that the model in six months has surpassed last year's total with more than 1,100 vehicles to spare.

As for the Leaf, Nissan has said all along that both deliveries and sales would pick up once U.S. production started up at the Japanese automaker's Tennessee plant later this year.

Still, the sales pattern for the Leaf is disconcerting. While June marked a 4.9-percent uptick from May, it also represented a 69-percent plunge from year-earlier sales. For the first half of the year, Nissan sold 3,148 Leafs in the U.S., down 19 percent from the first half of 2011.