The organization has hired a dozen professional trackers like Mr. Fielding, outfitted them with the latest high-tech cameras and computers and positioned them in key states where Republican candidates are busy chattering away to voters. If all works as planned, incriminating moments captured by American Bridge will quickly become part of the political bloodstream.

Combined with a team of 20 researchers in a Washington “war room” that has a large rack of computer servers, the effort is part of a push by Democratic groups to bolster their opposition research. Republicans also have trackers, but so far have not assembled the kind of centralized video archive of political caught-on-tape moments that their rivals envision…

American Bridge is the creation of David Brock, the founder of Media Matters for America, which critiques conservative media outlets, particularly Fox News. Once a leader of right-wing attacks on Mr. Clinton, Mr. Brock is a liberal convert, now seeking ways to have a bigger impact…

By tapping into his network of Media Matters donors, American Bridge officials say they have raised several million dollars and hope to bring in $15 million to make the group part of a new liberal infrastructure that will last far beyond the 2012 cycle.