Likely Democratic nominee Barack Obama has begun a top-secret search for a running mate, fresh signs that the general election campaign is well under way and the primary race against Hillary Rodham Clinton is basically over.

Obama 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.

The campaign did not want to discuss the effort because they are still engaged in a fading primary campaign against Hillary Rodham 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 61 delegates needed to clinch the nomination according to the latest Associated Press count. 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 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 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. Continued...