Obama secretly searches for running mate

WASHINGTON, May 23 (UPI) -- Campaign officials for Sen. Barack Obama, the likely Democratic U.S. presidential nominee, are secretly looking for a running mate, party officials said.

The campaign has begun to collect open-source research dossiers on a number of potential running mates, CBS News reported Friday. Sources told CBS the Obama campaign will begin vetting potential choices soon.

The Illinois senator asked former Fannie Mae CEO Jim Johnson to begin screening potential vice presidential picks, Democratic officials said. Johnson did the same job for Democratic presidential hopefuls John Kerry in 2004 and Walter Mondale in 1984.

The campaign is not publicly discussing the effort. Obama has refused to discuss possible running mates during the primary-and-caucus season.

"We're not commenting about this process," said Obama spokesman Bill Burton.

Just three primaries remain for Obama, D-Ill., and Sen. Hillary Clinton, D-N.Y. -- Puerto Rico, South Dakota and Montana. Voting ends June 3.

Obama needs 70 delegates to clinch the nomination, CBS' delegate tally indicates. Obama has 1,956 delegates to Clinton's 1,773 delegates. Under party rules, 2,026 delegates are required to secure the nomination.