On the eve of Bob Dole's announcement of his vice presidential running mate in 1996, John McCain knew he was under serious consideration. But he was on an ill-timed trip to Hawaii — without a cell phone.

As he tells it, he spent the entire time within close range of his hotel room, waiting for a call that never came. He learned Dole passed him over for Jack Kemp when he turned on the television news.

Now, with the Republican nomination virtually sewn up, McCain is facing a barrage of questions about whom he might choose as a running mate. Perhaps because of his own public vetting years ago, the Arizona senator is being uncharacteristically tight-lipped.

He frequently waves off queries with a joke that the vice president has just two duties: casting tie votes in the Senate and inquiring daily about the health of the president. But that hasn't stopped speculation about his frequent companions on the campaign trail and those who have made the invitation list for weekend retreats to the candidate's cabin outside Sedona, Ariz.

Many people believe that voters' concerns about McCain's age — he will be 72 on inauguration day — means his choice for the No. 2 spot will carry a great deal of weight.

"By the time this election gets around, everyone is going to know he (would) be the oldest president ever sworn in," Republican consultant Scott Reed said. "It's a concern and it has to be addressed."

But there is little consensus within the party about what issue will define McCain's choice. Should his team look to a candidate who could shore up his economic credentials? Should he choose a partner who could help allay suspicions among some conservatives that McCain is too liberal? Or does he have the latitude to choose a candidate who might help broaden the appeal of the Republican Party?

Reed, who was Dole's campaign manager and helped orchestrate the surprise choice of Kemp in 1996, said McCain will look for "a good, strong conservative" with a record of governing who could complement the ticket "both from a generational standpoint (and) a geographical standpoint."

Many conservatives view the selection process as McCain's opportunity to earn their confidence, said David Keene, chairman of the American Conservative Union.

"A lot of conservatives fear he's going to change (the party) in some way and redraw it with them on the outside looking in," Keene said. "If you select the right person, you go a long way toward solving that problem."

Ken Duberstein, a former chief of staff to President Reagan, says the field is open. "Does the right wing have veto power? The answer is no," Duberstein said. "Conservatives have a role to play, but it is not to dictate who the vice presidential candidate is."

Many people believe McCain will consider former Massachusetts Gov. Mitt Romney — whose experience running an investment capital enterprise could add economic heft to the ticket — even though the two had a testy relationship when they were rivals for the nomination. McCain also is expected to consider onetime White House hopeful Mike Huckabee, a skilled campaigner who could draw evangelicals. Huckabee, however, was derided widely by economic conservatives over his record on taxes when he was governor of Arkansas.

Several charismatic governors with close ties to McCain are getting attention as well: Charlie Crist of Florida, Tim Pawlenty of Minnesota and South Carolina's Mark Sanford.

Reed predicted Crist, 51, would be "on McCain's short list of three or four."

Susan MacManus, a political science professor at the University of South Florida, said Crist has "credentials in a lot of key policy areas" and is "very populist, people-oriented, kind of a sunny personality—it's probably a nice complement to McCain."

But, MacManus noted, he is viewed with suspicion in some conservative circles in Florida because of his nuanced view on abortion and his support for civil unions and for the expansion of stem-cell research.

Pawlenty, 47, is an early McCain supporter who won the Minnesota governorship in 2002 by advocating the Republican Party should be the party of "Sam's Club, not the country club." His unassuming demeanor—he likes to play in pick-up hockey games as he travels around the state—and commitment to fiscal restraint has lead to strong approval ratings.

Indeed, University of Minnesota political science professor Lawrence Jacobs said Pawlenty "is one of the most capable politicians for presenting himself as reasonable and likable." He won accolades in his party by taking a no-tax pledge when he ran in 2002 (although he did not repeat the pledge last cycle) and has vetoed a number of popular bills, including a recent transportation bill because of his opposition to tax hikes, Jacobs said.

"He's battling the Legislature and yet his approval ratings are pretty strong," Jacobs said.

Pawlenty will serve as host of the Republican convention later this year, but Jacobs and others have questioned whether the governor would be able to deliver Minnesota for McCain in November. Pawlenty defeated his Democratic opponent in the 2006 election by just 1 percent.

There is much lobbying among conservatives for Sanford, 47. The former three-term congressman is known for his political showmanship—from sleeping in his congressional office to save taxpayer money (he sent his housing allowance back to the federal treasury) to carrying two piglets into the statehouse to protest "pork barrel" spending in 2004.

But the South Carolina governor has proved to be politically divisive.

Time magazine ranked him as one of the nation's worst governors in 2005, in part because of the state's high unemployment rate. At the same time, Sanford's zeal for limited government led the libertarian Cato Institute to rank him as one of the nation's best governors.

Neal Thigpen, a political science professor at Francis Marion University in Florence, S.C., said Sanford has spent most of his time as governor "at war with the Republican-controlled state House and state Senate over spending."

Besides, Thigpen said, McCain is unlikely to pick a running mate from a state that he could "carry away in a hand basket."

Another possible vice presidential choice popular in conservative circles is fiscal hawk Rob Portman, 52, a former Ohio congressman who served for 14 months as President Bush's director of the Office of Management and Budget.

"Portman probably brings a lot to the table—congressional experience, executive experience, somebody who has been focusing on the economy (who is) from a swing state," said Jennifer Duffy, senior editor of the nonpartisan Cook Political Report. But a drawback for Portman and a number of the candidates mentioned, she said, is that none is a "known" quantity.

One disadvantage McCain must deal with in narrowing his list of possible running mates is not knowing who his Democratic opponent will be. Behind the scenes, the campaign will be doing a great deal of research and polling to weigh the drawbacks of various candidates.

But at this early stage, Reed said, it's "premature for McCain and team to really know what they need."

One of the campaign's most daunting tasks will be vetting the candidates to avoid any surprises.

"You have to look around every little corner and in every nook and cranny," Duberstein said. It will be important for McCain, he said, to choose a search leader who's "been around the track several times and who knows where skeletons usually are — what to ask and how to ask."