Republican presidential candidate and former Massachusetts Gov. Mitt Romney holds a towel next to his, wife Ann, before an NBA first-round playoff series basketball game between the Boston Celtics and the Atlanta Hawks in Boston, Sunday, May 6, 2012. (AP Photo/Michael Dwyer)

NEW YORK - @BarackObama is on Twitter. So is @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 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 - a study by the Pew Research Center found that about 13 percent of American adults have joined the site - 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 - it's the indirect message."

No one believes the campaign will be won or lost on Twitter; 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 - from the broadcaster to the public. Never before has a grassroots technology like Twitter given both voice and power to millions, and given candidates a real-time way to monitor the effects of their messages and recalibrate on the fly.

And that means an ever-changing campaign narrative for 2012.

Four years ago, Twitter was in relative infancy and just 1.8 million tweets were sent on Election Day 2008. Now, Twitter gets that many approximately every eight minutes.

Obama's 2012 State of the Union address drew 800,000 tweets, Twitter said. And tweets mentioning Rick Santorum jumped from 10-20 per minute to more than 2,500 tweets per minute when news broke that he was suspending his bid for the Republican presidential nomination.

Both the Obama and Romney campaigns have actively embraced Twitter, using it to communicate directly with supporters and, more importantly, drive the political conversation in a way that reaches far beyond the site. They're also mindful of the hazards of Twitter, designating war room staffers to monitor the site for problems to address or gaffes from their rivals to exploit.

"Our team understands that the most important issues in this campaign are jobs and the economy, not the Twitter controversy of the day," Romney spokesman Ryan Williams said. "But we need to be on top of everything and monitor every aspect of this race. Twitter helps us keep our finger on the pulse of the fast moving pace of new media."

Twitter's impact was on vivid display last week.

Obama, warning a college audience that interest rates on their federal student loans could double if Congress doesn't act soon, urged students to make their voices heard on Twitter.

"Tweet them! Teach your parents how to tweet!" Obama said, asking them to add the hash tag (hash)dontdoublemyrate.

The Obama campaign's introduction of a campaign tool named Julia also showcased what both parties will do to get the last word on Twitter.

Obama's team unveiled an interactive slideshow on its website showing how a hypothetical woman named Julia would benefit from the president's policies throughout her life.

"Follow Julia from age 3 to 67," the Obama campaign tweeted with a link to the tool - all but guaranteeing a level of buzz among Twitter users that then spilled into other social media and into reporters' stories.

Republicans, for their part, moved quickly on Twitter to respond - tying Julia to the persistent weak economy.

"Did u tell (hash)Julia how much debt you left her?" Republican National Committee spokesman Sean Spicer tweeted.

"Based on today's bad unemployment report, it appears that Julia has given up looking for work," former George W. Bush spokesman Ari Fleischer tweeted after Friday's unemployment figures showed tepid job growth.

Twitter's warp speed presents both an opportunity and a challenge to campaigns ever vigilant about maintaining message discipline.

The Romney campaign sought to seize advantage after Democratic strategist Hilary Rosen remarked that the presumptive Republican presidential nominee's wife, Ann, a stay at home mother of five sons, had "never worked a day in her life." Polls show Romney lagging badly among women voters, and his advisers have sought ways to close the gender gap.

After Rosen's comments on CNN quickly exploded - on Twitter - the Romney campaign launched a Twitter feed from the candidate's wife.

"I made a choice to stay home and raise five boys. Believe me, it was hard work," (at)AnnDRomney said in her first-ever tweet. "All moms are entitled to choose their path," came her second.

Ann Romney's engagement on Twitter quickly earned her several thousand followers on the site. But it also scored news headlines and helped cast Democrats as unsympathetic to women who stay home with children - a score for the Romney campaign that went far beyond the Twitter audience.

Twitter has also caused both campaigns plenty of headaches.

The Obama team was forced onto defense during the Rosen controversy, even though she has no connection to the president's re-election effort. The campaign deployed Michelle Obama to push back on Rosen.

"Every mother works hard and every woman deserves to be respected," the first lady tweeted.

In February, Romney delivered an economic speech at Ford Field in Detroit - a must-win primary state for the Michigan native as he battled rival Santorum for the GOP nomination.

But hours earlier, reporters began tweeting photos of the nearly empty football arena and the small section of it reserved for Romney's event. By the time the former Massachusetts governor delivered the address, the ill-chosen venue had become the story instead.

Twitter also contributed to the resignation of a Romney foreign policy spokesman last week. Richard Grenell stepped down in part because of caustic tweets he had sent about a host of public figures including MSNBC host Rachel Maddow and Newt and Callista Gingrich.

Indeed, Tom Rosenstiel, director of the Pew Research Center's Project for Excellence in Journalism, said the snark factor is one of Twitter's biggest pitfalls.

"You're more likely to be embarrassed by what's said on Twitter than to be praised," Rosenstiel said. "The things that go viral tend to be jokes and tend to be mocking."

But, he added, "Twitter has this quality of being an alert system that elevates it above the number of people using it."