US:VICE-PRESIDENTIAL candidates have rarely made much difference to the outcome of US elections. But John McCain's choice of running mate could be an exception.
The Arizona senator, now Republican nominee, will be 72-years-old on inauguration day next January. If elected, he would be the oldest first-term US president in history.
While Mr McCain insists he is fit and healthy, his age makes it more important than usual for his running mate to be considered capable of becoming commander-in-chief at a heartbeat's notice.
Economic expertise is likely to be another requirement given Mr McCain's self-declared weakness on the issue. Executive experience, preferably outside Washington, would be another advantage.
As a Republican moderate, he is under pressure from conservatives to pick someone from the right, such as Rick Perry, the governor of Texas, or Sam Brownback, senator for Kansas.
Many experts believe Mr McCain needs a conservative running mate, preferably from the south, to unify the fractured Republican base. But others argue he has most to gain from picking a fellow moderate, such as Chuck Hagel, senator for Nebraska, or Joseph Lieberman, the independent senator for Connecticut, who would reinforce his appeal among swing voters.
Another option might be Michael Bloomberg, the New York mayor and billionaire media tycoon, who recently ended speculation that he might make his own run for president.
Norm Ornstein, a political scientist at the American Enterprise Institute, doubts Mr McCain would dare test the loyalty of Republicans by steering too close to the centre. He predicts the senator will look to someone conservative enough to reassure the base without alienating independents.
Mr McCain is likely to wait until he knows for sure who his Democratic opponent will be before making a decision. If it turns out to be Barack Obama, a young and charismatic running mate may be needed to lower the average age of the ticket and combat the Illinois senator's star power. Alternatively, he might opt for someone with experience and gravitas to sharpen the contrast with Mr Obama's rawness. Colin Powell, the former secretary of state, would be an obvious choice - but he is thought unlikely to want the job.
If Hillary Clinton was the opponent, Mr McCain might opt for a woman such as Sarah Palin, the popular young governor of Alaska, or Kay Bailey Hutchinson, senator for Texas. As an African-American woman, Condoleezza Rice would help neutralise the appeal of either Mr Obama or Mrs Clinton, but would also make it easier for the Democrats to link Mr McCain to the failures of the Bush administration.
Of the men who challenged Mr McCain for the nomination, Mike Huckabee would help mobilise evangelical Christians but alienate moderates and economic conservatives. Rudy Giuliani would alarm social conservatives; Fred Thompson, at 65, is probably too old. Mitt Romney would provide the executive leadership that Mr McCain lacks, but the men were bitter rivals on the campaign trail.
There is one candidate who ticks almost every box. He is relatively young. He was a popular two-term governor of a crucial swing state. He is solidly conservative but also appealed to moderates and Hispanics. His name? Jeb Bush, the former Florida governor and brother of the president.
"If it wasn't for his name, he would be the perfect candidate," says Paul Light, an expert on the vice-presidency at New York University.