WASHINGTON - Likely Democratic nominee Barack Obama has reportedly begun a closely guarded search for a running mate, according to party sources -- fresh signs that the general election campaign is well under way and the primary race against Hillary Rodham Clinton is considered basically over.

Obama has asked former Fannie Mae CEO Jim Johnson to begin vetting potential vice-presidential picks, Democratic officials said Thursday, speaking on condition of anonymity. Johnson did the same job for Democratic nominees John Kerry in 2004 and Walter Mondale in 1984.

Obama refused to acknowledge Johnson's role when The Associated Press asked the Illinois senator about it in the Capitol on Thursday.

"I haven't hired him. He's not on retainer. I'm not paying him any money. He is a friend of mine. I know him,'' Obama said. "I am not commenting on vice-presidential matters because I have not won this nomination.''

Vice-presidential searches are usually closely held secrets, but Obama campaign officials say the current effort is being handled by a particularly tight circle of advisers.

The campaign did not want to discuss the effort because they are still engaged in a primary campaign against Hillary Rodham Clinton, with three primaries left in Puerto Rico, South Dakota and Montana.

The voting ends June 3 and 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 sources say that with just 61 delegates needed to clinch the nomination, they are taking behind-the-scenes steps to move toward the general election campaign.

According to the latest count by The Associated Press, Obama has 1,965 delegates to Clinton's 1,780, with 2,026 required to secure the party's nod under Democratic National Committee rules.

The Obama campaign is rapidly adding to its campaign staff, both at the headquarters and in general election swing states.

Obama has been travelling to some of those battlegrounds -- Missouri, Michigan, Iowa and Florida in the last nine days -- while the campaign is registering voters across the United States for the November presidential election. Top Obama organizer Paul Tewes is in discussions to take over the Democratic National Committee.

It's 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 several months since the GOP primary race wrapped up in early March.

McCain is hosting at least three Republicans mentioned as potential vice-presidential running mates at his Sedona, Ariz., home this weekend -- Florida Gov. Charlie Crist, Louisiana Gov. Bobby Jindal and former Massachusetts governor Mitt Romney. A top aide said it is a social event with more than two dozen guests not meant for vice-presidential vetting.

Obama's campaign refused to talk about who was being considered, but some in the party are calling for him to pick Clinton.

Clinton spokesman Howard Wolfson said Thursday that "there have been no discussions with the Obama campaign about Senator Clinton being the VP.''

Other possible options are governors such as Arizona's Janet Napolitano, Kathleen Sebelius of Kansas and Tim Kaine of Virginia; foreign policy experts like former Georgia senator Sam Nunn, Connecticut Senator Chris Dodd or Delaware Sen. Joe Biden; or other senators such as Missouri's Claire McCaskill and Virginia's Jim Webb.

He could look outside the party to people such Republican Senator Chuck Hagel, a critics of the war in Iraq, or independent New York Mayor Mike Bloomberg. Or he could look to one of his early prominent supporters such as former senator Tom Daschle of South Dakota or 2004 vice-presidential nominee John Edwards. Or he could try to bring on a Clinton supporter like Indiana's Evan Bayh.

Johnson's role running the vice-presidential process was first reported on TheAtlantic.com.