Democratic officials say Barack Obama has begun a top-secret search for a running mate. Democratic officials said Thursday the party's likely nominee has asked former Fannie Mae CEO Jim Johnson to begin vetting potential vice presidential picks. Johnson did the same job for Democratic nominees John Kerry in 2004 and Walter Mondale in 1984. The Democratic officials spoke on a condition of anonymity about a process that the campaign wants to keep quiet. Vice presidential searches are usually closely held secrets, but Obama campaign officials say the effort is being handled by a particularly tight circle of advisers. The campaign also does not want to discuss the effort because they are still engaged in a fading primary campaign against Hillary Rodham Clinton. Obama has repeatedly declined to discuss possible running mates while the primary is ongoing.