Counteracting a Vulnerability
Sam Nunn, 69, is still respected as former chairman of the Senate Armed Services Committee, even though he retired from the Senate in 1997. He could also help Obama carry Georgia. But his views on social issues such as gay rights may be too conservative.
Mitt Romney is a fit 61, and his business experience makes him a Wall Street favorite. The former Massachusetts governor was vetted by the press during his nomination run. But he has been tagged a flip-flopper, and his relationship with McCain has not been warm.
James Webb, 62, brings the tantalizing prospect of helping Obama carry Virginia, a traditional GOP state, which Webb represents in the Senate. He also brings cross-party appeal as a former Navy secretary under Ronald Reagan, but he strikes some as too temperamental.
Rob Portman has business experience as former head of the Office of Management and Budget. The Cincinnati lawyer, 52, is one of the party's new leaders. The former congressman would help put a big swing state in play for McCain. But he was a Bush appointee.
Kathleen Sebelius, 60, would appeal to women and reformers. As governor of Kansas, she has worked well with Republicans and is seen as an agent of change. But her choice might offend those who believe Hillary Clinton should share the ticket with Obama.
John Thune became a conservative hero when he beat incumbent South Dakota Sen. Tom Daschle. Thune, 47, endorsed McCain early on and would help with voters in the Midwest and the West. But he is a senator with little national stature.
The Also Ran
Hillary Clinton, 60, received 18 million votes in the primaries and demonstrated strong appeal to women and working-class Democrats. She fell short of the nomination but became an effective campaigner. However, it's unclear how she would adjust to being Obama's No. 2.
Mike Huckabee energized the conservative Christian base. The minister and former Arkansas governor won eight contests before ceding the race to McCain. An appealing campaigner, Huckabee, 52, might not pass the "could he be president" test.
The Wild Card
Chuck Hagel, 61, would be a dramatic but risky choice for Obama. As a Republican, the Nebraska senator would represent a strong overture to the GOP. Many Democrats admire Hagel's strong opposition to the Iraq war. But his views on social issues may be too conservative.
Meg Whitman would be a groundbreaking choice for McCain. As eBay head, she oversaw expansion from $47 million in annual revenue and 30 employees to $7.6 billion and 15,000 workers. Retired at 51, she's a political neophyte, which could lower her stock.