Automaker discounts and incentives prompted millions of consumers to visit auto manufacturer sites in March 2010, while political news and information sites recorded sizeable gains in traffic as Americans tuned into the debate over healthcare reform, according to data from the comScore Media Metrix service.

• Auto-Manufacturer websites were the top-gaining category in March, attracting 30.3 million unique visitors, up 23% from the previous month. General Motors led the category with 7.6 million visitors, up 32%, followed by Hyundai Motors with 5.4 million visitors, a 222% surge from February levels. Toyota attracted 5.1 million visitors, followed closely by Ford Motor Company with 5.0 million (up 3%). Chevrolet grew 71% to 4.4 million visitors, while Honda attracted 3.6 million visitors.

• News/Information-Politics websites attracted 20.5 million unique visitors during the month, up 18% from February levels. PoliticsDaily attracted 5.3 million visitors, while The Economist Group grew 35% to 2.7 million visitors and Politico grew 10% to 1.8 million visitors. The number of visitors to BarackObama.com more than doubled during the month, growing 133% to 1.5 million visitors.

• Business/Finance-Online Trading websites attracted 11.5 million unique visitors in March, up 13% from the previous month as the stock market continued to rebound. Fidelity Investments was the top investment site with 3.5 million visitors, up 10%. Scottrade Sites ranked second with 1.6 million visitors (up 7%), followed by ShareBuilder with 1.5 million visitors (up 20%), and ETrade Financial Network with 1.3 million visitors (up 25%).

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comScore also released March data on the top gaining online properties, led by Unister, Kraft, and FunnyorDie: