Sources told CBS News the Obama campaign will begin vetting potential choices soon.
CBS News chief political consultant Marc Ambinder reports that the campaign is starting to collect dossiers based only on open source research on a large number of potential picks so that Obama has something to read when he starts to think about the choice. (Read more from Ambinder on the process and potential picks.)
Obama also has asked former Fannie Mae CEO Jim Johnson to begin vetting potential vice presidential picks, Democratic officials said Thursday. 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 Captiol 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."
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.
By June 4, the day after Democrats finish voting, the campaign hopes to have a full team in place, Ambinder reports.
"He wants this done right," one person who is privy to the candidate and campaign's thinking on the matter told Ambinder. "He takes this very seriously."
The campaign did not want to publicly discuss the effort because they are still engaged in a fading primary campaign against Clinton, with three primaries left in Puerto Rico, South Dakota and Montana. The voting ends June 3. 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 70 delegates needed to clinch the nomination according to the latest CBS News count. Obama has 1,956 delegates to Clinton's 1,773, 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 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's all part of an effort to lay the groundwork for an aggressive kickoff to a general election campaign. Republican has a head start and has been building his effort for several months since the GOP primary race wrapped up in early March.
Horserace: The Mating Dance Begins
CBSNews.com's Vaughn Ververs provides some helpful ground rules in watching the process by which running mates are chosen.
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 Gov. Mitt Romney. A top aide said it's a social event with more than two dozen guests not meant for veep vetting.
Obama's campaign refused to talk about who was being considered, but possible options are Clinton; governors such as 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 such as Missouri's Claire McCaskill and Virginia's Jim Webb.
He could look outside the party to people such as war critic and Nebraska Republican Sen. Chuck Hagel or independent New York mayor Mike Bloomberg. Or he could look to one of his early prominent supporters such as former Sen. Tom Daschle of South Dakota or try to bring on a Clinton supporter like Indiana's Evan Bayh.