In a sign of the strengthening economy, more than 15 million new cars and light trucks will be sold in the United States this year, up from 14.4 million in 2012. Among them will be a record number of electric vehicles, though overall volumes remain low as consumers slowly grow comfortable with the new technology.

"There was an expectation that electric vehicles would take off very quickly, but this is a process," said auto industry analyst Alan Baum. "It's a process for consumers and it's a process for the automakers. You can't turn this market on with a switch. It's a market that will slowly increase."

Nissan says it is on track to sell roughly 22,000 all-electric Leafs in the United States this year, which surpasses the 19,512 Leafs sold in the U.S. in 2011 and 2012 combined.

The Leaf, which has a base price of $28,800 before federal or state tax credits and has a range of about 75 miles per charge, was first introduced three years ago. California still accounts for roughly a third of all sales, and cities such as San Francisco, Seattle and Atlanta have emerged as major markets.

Sam Ori, executive vice president of the Electrification Coalition, a nonpartisan advocacy group, notes that plug-in electric vehicles are just in their third year.

"There are now 17 models on the market," Orsi said. "It's not just the Volt and the Leaf."

Though several major automakers have recently introduced electric vehicles -- including the Fiat 500e, Honda Fit EV and Ford Fusion Energi -- the top three sellers in the electric vehicle and plug-in hybrid electric vehicle category remain the all-electric Nissan Leaf and Tesla Model S sedan, and the gas-and-electric Chevy Volt.

General Motors is expected to sell more than 21,000 Chevy Volts this year. Palo Alto-based Tesla Motors (TSLA) does not disclose monthly sales figures but said in a recent regulatory filing that it is making 550 cars a week at its Fremont factory and will deliver 21,500 Model S sedans in 2013, which includes some initial deliveries to Europe and Asia.

"Elon Musk has done a good job of selling the Model S as a vehicle, not just an electric vehicle," Baum said.

Nissan executives say they are pleased with the Leaf's sales, noting that the market has moved beyond "early adopters" who were primed to embrace electric transportation. As the Leaf begins to reach mainstream consumers, Nissan is seeing an organic sales pattern emerge in which owners of Leafs convince their friends and neighbors to buy the vehicle.

"This is a major technology shift in many people's minds," said Erick Gottfried, Nissan's director of EV sales and marketing. "We're beyond early adopters and have reached a point where we are selling to a practically minded consumer who is looking at the monthly economic savings of driving the car."

Some analysts say lower gas prices in recent months, along with improved fuel efficiency for gasoline-powered cars, have tempered consumer demand for alternative-fuel vehicles. According to a Fuel Economy Trends report recently released by the U.S. Environmental Protection Agency, new automobiles sold in model year 2012 averaged a record-high 23.6 miles per gallon; for 2013, they are expected to average 24 mpg.

"Low and falling gas prices do not help the electric vehicle market," said Jessica Caldwell, an analyst at Edmunds.com. "There tends to be a lot of exposure for advanced drive vehicles in the media when gas prices are high, and that exposure leads to increased consideration."

Contact Dana Hull at 408-920-2706. Follow her at Twitter.com/danahull.

Projected 2013 U.S. sales of top-selling plug-in electric vehicles
1. Nissan Leaf: 22,000
2. Chevy Volt: more than 21,000
3. Tesla Model S: at least 21,500*
* Tesla's figures include international sales, though most of its cars are sold in the U.S.
Source: Mercury News reporting