As potential voters in New Hampshire and Iowa scan the Internet, they probably are seeing ads for Republican Mitt Romney and President Barack Obama alongside deals for shoes and holiday gifts.
The campaign ads will then follow those voters around the Web, popping up on news sites, Google searches and on social networking sites like Facebook.
Online advertising, once used primarily as a way to reach young and heavily wired consumers, has emerged as an essential communications tool in the 2012 presidential contest. While few expect Web ads to supplant television commercials anytime soon,...