THEY used to call it “opposition research”, but in the modern digital world of Google, MySpace and YouTube, the candidates running in America’s mid-term elections have given their efforts to undermine rivals another name: “Gotcha!”.

In several key races for next month’s state and national elections, candidates have fallen foul of a sneaky trend in subversive campaigning. A new breed of persistent and inventive political operative is deploying across the country, digital video cameras in hand, hoping to record a fatal gaffe by a candidate caught off-guard.

The cameramen are known as “trackers” — campaign agents who are paid to pursue a rival candidate and record everything he says or does. They have already claimed several prominent scalps as politicians hurriedly adjust to a new age of round-the-clock scrutiny and the threat of instant internet infamy.

“It’s ‘gotcha’ politics,” complained Senator Conrad Burns, a Republican whose bid for a fourth term in the generally conservative state of Montana was proceeding uneventfully — until a pesky Democrat named Kevin O’Brien started following him with a Sony Digicam.

O’Brien, 24, has driven 17,000 miles in pursuit of candid moments at Burns’s election meetings. He has been rewarded with a series of video scoops that, thanks largely to the YouTube video-sharing site, has attracted national headlines and sabotaged Burns’s Senate campaign.

One clip showed the 71- year-old senator appearing to fall asleep during a meeting on farm policy. Entitled “Conrad Burns’s Naptime”, the one-minute clip was watched more than 75,000 times on YouTube and was shown by several television networks.

Then there was the time O’Brien caught Burns warning his constituents about people “who drive taxicabs in the daytime and kill at night”.

While Burns may have been in trouble anyway — Republicans have been losing ground in the West — O’Brien’s tapes have helped the Democrat challenger Jon Tester to edge ahead in a seat that may decide which party controls the US Senate next month.

Nor is Burns the only Republican to have been stung during the first US election in which home-produced videos have been so widely available. Arnold Schwarzenegger, the California governor seeking re-election, was caught in a casual moment admitting that his budget plan would not end the state’s huge deficit, as he had earlier implied.

Schwarzenegger’s “trackers” promptly responded with a “gotcha” moment of their own — they produced a clip showing the governor’s Democratic rival Phil Angelides seeming to contradict his previous statements on health insurance.

Both the California candidates subscribe to a new high-tech service that alerts campaign staff whenever any televised California newscast mentions their rival’s name. Within seconds, staff can review the mention and compare any statements made with an archive of previous recordings.

Contradictions are promptly posted online and the link is then e-mailed to journalists, in the hope that the gaffes will become mainstream news.

“I now know that I cannot stop and talk to two young people in the hallway without the potential of my words being on MySpace or some blog or YouTube by the end of the day,” said Bob Mulholland, a consultant to the Democratic party. “There are no longer private conversations.”

Another Democratic tracker, SR Sidarth, caused a political row when his persistent filming of Senator George Allen of Virginia provoked the Republican into calling him a “macaca” — a racial slur in parts of Latin America. Sidarth was actually of Indian origin, but Allen was accused of racism and began to slip in the polls.

Operatives on both sides say that “gotcha” moments may well prove decisive in future campaigns. “It’s here and we have to deal with it,” said an aide with the Burns campaign.

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