BOCA RATON, Fla. - The Democratic presidential front-runner, Sen. Barack Obama, took his first step toward selecting a running mate, asking a prominent supporter to begin the search process, while his likely GOP opponent, Sen. John McCain, prepared to spend the weekend with three potential vice presidential picks.

Former Fannie Mae chief executive Jim Johnson will head Obama's vetting team, Democratic sources said. Johnson played a similar role for nominees John Kerry and Walter Mondale and joined the Obama campaign last year as a vice chairman.

Obama and his aides described the move as only a preliminary exercise, and stressed that they remained focused on the primary battle with Sen. Hillary Rodham Clinton.

"I haven't hired him. He's not on retainer. I'm not paying him any money. He is a friend of mine," Obama said outside the Senate, where he returned yesterday to vote for an Iraq funding bill. "I am not commenting on vice presidential matters because I have not won this nomination."

Aides to McCain are working quietly behind the scenes to vet possible running mates and were equally reluctant to talk about it. Republicans familiar with the process said campaign aides were conducting background checks on what one called "a long list" that reached double digits.

McCain is looking at a range of people, including Govs. Charlie Crist of Florida, Tim Pawlenty of Minnesota, and Bobby Jindal of Louisiana; former Rep. Rob Portman of Ohio; and one of McCain's primary opponents, former Massachusetts Gov. Mitt Romney. Crist, Jindal and Romney will visit McCain's ranch in Sedona, Ariz., this weekend, though the senator's top aides insisted the visits weren't part of a formal search.

Former Pennsylvania Gov. Tom Ridge was among the prospective running mates invited for the weekend, but he told Allentown's Morning Call newspaper yesterday that he could not attend because of a previous commitment in Europe involving his strategic-consulting business, Ridge Global. He said he was "glad to be invited" and would campaign with McCain in June.

Obama aides are guarding carefully against offering any appearance of overconfidence as he and Clinton campaign through the final June 3 primaries in Montana and South Dakota. "We've got a little more work to do," he told Jewish voters at a Boca Raton synagogue, to whom he promised "an unshakable commitment to Israel's security."

Clinton's campaign kept its focus on breaking the Florida and Michigan delegate impasse, viewing the seating of those delegations as her best chance for remaining viable.

As the primaries wind down, Obama is trying to balance his contest with Clinton and his desire to take on McCain directly and frame the general-election debate. Early next week, Obama will visit New Mexico, Nevada and Colorado, three swing states he hopes to win Nov. 4.

This week he campaigned in Florida, another swing state.

In Boca Raton, he was asked about his criteria for choosing a vice president. "My goal is to have the best possible government," Obama replied. "I am very practical-minded." He noted that one of his heroes, Abraham Lincoln, named political rivals to his cabinet. "How can we get this country through this time of crisis? That has to be the approach that one takes."

Among others, Obama is expected to look at seasoned statesmen such as Sen. Joseph R. Biden Jr. of Delaware and former Sen. Sam Nunn of Georgia. Sen. Jim Webb of Virginia is another potential prospect, a decorated Marine and former Republican with strong working-class support.

Some Obama insiders believe he also will consider female candidates, including Kansas Gov. Kathleen Sebelius, Arizona Gov. Janet Napolitano, and Sen. Claire McCaskill of Missouri.

McCain has said he wants a running mate who shares his beliefs and would be prepared to take over the presidency if needed. His team is also taking political calculations into consideration, including geography, age, and expertise on domestic issues.

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