Increasingly acting like the Democratic presidential nominee, Barack Obama is beginning to vet potential running mates, laying plans to take control of the party’s campaign apparatus and trying to overcome vulnerabilities exposed in the prolonged primary season.

Obama has not asserted the nomination is his, for fear of offending supporters of his rival, Hillary Rodham Clinton, who lags Obama among delegates to the party’s nominating convention but shows no signs of conceding the race. Still, his recent campaign stops and administrative moves show that his central focus is the November election.

The campaign has tapped James A. Johnson, an Obama fundraiser, to oversee the screening of potential vice presidential candidates, according to campaign aides.

Johnson, vice chairman of the merchant bank and private equity fund Perseus, worked as a top aide to former Vice President Walter Mondale. He helped Mondale vet potential running mates in his unsuccessful 1984 campaign and played a similar role for Massachusetts Sen. John F. Kerry, the 2004 Democratic nominee.

“He’s been through this VP thing many, many times,” Mondale said. He said Johnson “worked for me in a key position in the White House. He knows what it is, because he lived there.”

With only three more primary contests remaining — in Puerto Rico on June 1 and South Dakota and Montana on June 3 — Obama is also moving to solidify his position in states that will be battlegrounds in November.

Obama does not want to appear to be pushing Clinton from the stage, so he can remain well-positioned to win the votes of her supporters in the general election.

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