As US election day approaches, "Twitter bombs" are expected to fly as part of efforts to swing votes in a close race.

The "bombs," a form of spam, are unsolicited Twitter responses which are retweeted with the help of automated computer programs, or "bots", to get maximum distribution.

These are examples of cyber tricks which could surface in the final days of the US election campaign from candidates or, more likely, outside groups supporting a candidate or cause.

The "bomb" technique is as old as Twitter, and is similar to e-mail spam. Both can be used for commercial or political causes.

Experts say it leads those receiving the messages to believe there is a groundswell, when in fact, the trend is being manipulated by those generating the message.

"This is a new way of propagating information or misinformation," said Panagiotis Metaxas, a Wellesley College computer scientist who studies social media.

"I suspect this will happen close to the final days of the election. With social media you can propagate information or misinformation fast."

-- Bomb campaigns as late game-changers --

Metaxas, who co-authored a report on the subject in Science Magazine, said that even though falsehoods can be debunked, "if you launch it close enough to the election you can keep enough confusion to last a couple of days."

Mexatas studied a case in the 2010 race for US Senate in Massachusetts, in which a "tweet factory" created in the final days of the campaign produced 1,000 tweets, which ended up being retweeted 60,000 times.

The campaign came from a conservative group in Iowa, in support of Republican Scott Brown, who eventually won the race.

"I cannot tell if it made a measurable difference in the election," Metaxas said. "But in a close race decided by a few hundred or a few thousand votes, this could be a factor."

"In times of political elections, the stakes are high, and advocates may try to support their cause by active manipulation of social media," Metaxas added.

In politics, Twitter manipulation can multiply messages from one person or group, making it appear to be a grassroots movements, a phenomenon known as "astroturfing."

"These are nefarious tactics," said Jeanette Castillo, a professor of communication at Florida State University who follows new media.

But Castillo said that over the long run, these efforts may be misguided, because it can hurt a candidate's "social media capital."

"These kind of tactics may be able to influence trends on Twitter, but I think they represent a basic miscalculation," she said. "Bots can't vote... or volunteer, or talk to their co-workers, family and friends."

Filippo Menczer, head of Indiana University's Center for Complex Networks and Systems Research, has set up a research project called "Truthy" in an effort to separate real groundswells from fake ones.

"We try to find out how it is that some information goes viral and some does not," he said.

"Fake accounts can give the impression of a grassroots movement when in fact it is just the coordinated effort of one person or one group."

Menczer's team uncovered a case in 2010 in which 20,000 tweets originated from two Twitter accounts in an effort to drum up support for Republican congressional candidates, and frequently mentioned House GOP leader John Boehner.

This effort likely used "bots" to get the messages out to a wider audience, he noted.

These efforts violate Twitter's terms of service, but spammers often disguise their messages by tweaking hashtags or other parts of the messages.

Twitter did not respond to an AFP query. But Menczer said Twitter would maintain a confidential algorithm in order to block efforts like "bombs" and "astroturfing."

-- 'Google bombs' --

Twitter bombs are not the only dirty tricks one can expect in cyberspace.

Another is the "Google bomb" or "search engine bomb" which can manipulate search results. Earlier this year, people searching Google for "completely wrong" got pictures of Mitt Romney.

Another showed pictures of former president George W. Bush in a search for "miserable failure" in 2003, until Google modified its algorithms.

A barrage of text messages in the past few days with anti-gay and false claims about Obama also came to light, drawing complaint from some groups.

These tactics are likely to grow as candidates and their supporters try desperation moves.

"It is an arms race," said Menczer.

"There are political or commercial reasons people have an incentive to build better systems (to get a message out) and Twitter has an incentive to build better countermeasures."

Menczer said the same techniques could be used to support, or thwart, political movements such as the Arab Spring uprisings.

"We have seen in Russia an attempt by a protest movement to use social media thwarted by junk" which drowned out the message, he said.

"I think it's something to worry about."