While many GOP leaders bang the drum against government subsidies for both makers and buyers of advanced-powertrain vehicles, the Carnegie Endowment has just put out a new report that says more federal and local incentives will be needed to ensure electric-drive vehicle sales gain momentum.

As it stands, plug-in vehicle battery costs will need to fall from about $700 per kilowatt hour to something like $325 in order for battery-electric vehicles to become price-competitive with conventional vehicles, the study said. The report also listed about two-dozen U.S. "vanguard cities" for plug-in vehicle progressiveness, including usual suspects San Francisco, Boston and Austin, as well as cities like St. Louis, Atlanta and Columbus, OH.

When it comes to cleaning the vehicle fleet, in addition to the stricter fuel-economy standards already proposed, the government will have to consider expanding policies that provide more incentives to plug-in vehicle makers while encouraging alt-fuel vehicle purchases in regions with cleaner electricity sources. President Obama has proposed increasing customer incentives, but that idea has not gained traction yet. Last year, plug-in vehicles accounted for about one in every 700 new light-duty vehicles purchased.

Want to read more from Carnegie's findings? Grab yourself a cup of coffee and check out the 53-page paper here.