Democratic officials say Barack Obama has begun a top-secret search for a running mate.Democratic officials said Thursday the party's likely presidential nominee has asked former Jim Johnson, the former CEO of US government-sponsored mortgage giant Fannie Mae, 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."We're not commenting about this process," said Obama spokesman Bill Burton.But they are taking behind-the-scenes steps to move toward the general election campaign, with just over 60 delegates needed to clinch the nomination.The Obama campaign is rapidly adding to its campaign staff, both at the headquarters and in general election swing states. Obama has been traveling to some of those battlegrounds - Missouri, Michigan, Iowa and Florida in the last nine days - while the campaign is registering voters across the country for the November vote. And top Obama organizer Paul Tewes is in discussions to take over the Democratic National Committee.It is all part of an effort to lay the groundwork for an aggressive kickoff to a general election campaign. Republican John McCain has a head start and has been building his effort for more than three months since the Republican primary wrapped up so much sooner.McCain is hosting at least three Republicans mentioned as potential vice presidential running mates at his Sedona, Arizona, home this weekend - Florida Gov. Charlie Crist, Louisiana Gov. Bobby Jindal and former Massachusetts Gov. Mitt Romney. A top aide said it is a social event with more than two dozen guests not meant for vetting a candidate for the vice presidency.Obama's campaign refused to talk about who was being considered, but possible options are Clinton; governors like Arizona's Janet Napolitano, Kathleen Sebelius of Kansas and Tim Kaine of Virginia; foreign policy experts like former Georgia Sen. Sam Nunn, Connecticut Sen. Chris Dodd or Delaware Sen. Joe Biden; or other senators like Missouri's Claire McCaskill and Virginia's Jim Webb.He could look outside the party to people like war critic Nebraska Republican Sen. Chuck Hagel or independent New York mayor Mike Bloomberg. Or he could look to one of his early prominent supporters like former Sen. Tom Daschle of South Dakota or try to bring on a Clinton supporter like Indiana's Evan Bayh.