BETH FOUHYNEW YORK (AP) â¿¿ (at)BarackObama is on Twitter. So is (at)MittRomney. And so are all the voters following the 2012 presidential contest, whether they know it or not. Candidates, strategists, journalists and political junkies have all flocked to Twitter, the social networking hub where information from the mundane to the momentous is shared through 140-character microbursts known as tweets. While relatively few voters are on Twitter â¿¿ a study by the Pew Research Center found that about 13 percent of American adults have joined the site â¿¿ it's become an essential tool for campaigns to test-drive themes and make news with a group of politically wired "influencers" who process and share those messages with the broader world. Put simply: When a voter is exposed to any information related to the presidential contest, chances are it's been through the Twitter filter first. "The subset of people on Twitter may be relatively small, but it's a politically engaged audience whose influence extends both online and off," said Heather LaMarre, a University of Minnesota communications professor who studies social media. "It's not the direct message that has the biggest influence on people â¿¿ it's the indirect message." No one believes the campaign will be won or lost on Twitter â¿¿ it's just one slice of an enormous communication effort the presidential campaigns are waging in cyberspace. But with a well-timed 140-character blast, candidates influence coverage, respond to charges or reinforce talking points. This, of course, is not the first time technology has changed the way campaigns are conducted. Radio, TV and the Internet all prompted campaigns to adapt, giving both more avenues to reach voters and more control of their message. But radio and television are top-down mediums at heart â¿¿ from the broadcaster to the public. Never before has a grassroots technology like Twitter given both voice and power to millions â¿¿ and given candidates a real-time way to monitor the effects of their messages and recalibrate on the fly.