With the gruelling task of winning his party's presidential nomination now behind him, Senator John McCain - and political pundits throughout the US - can begin to ask the next key question.
Mr McCain may pick someone who offers things that he cannot
Who will he choose as his running-mate?
"We have just begun that process," said Mr McCain on Wednesday, the day after he had won enough delegates to clinch the nomination.
The person Mr McCain chooses will appear on the ballot in November alongside him, and will become his vice-president if the pair win the election.
Generally speaking, candidates tend to pick running-mates who fulfil one of two criteria.
Most often, they are chosen because they possess qualities which the candidate himself lacks, or because they represent political or geographical constituencies which the candidate finds it difficult to appeal to.
This is known as "balancing the ticket".
Healing the wounds
So the young and relatively inexperienced north-easterner John F Kennedy opted for Lyndon Johnson, a Texan, who had served for 23 years in the US Congress.
And the seasoned politician George H W Bush selected Dan Quayle, the junior senator from Indiana.
Charlie Crist's endorsement helped Mr McCain in the Sunshine State
Balancing the ticket can be a useful way to heal any wounds caused by a divisive primary campaign.
Occasionally, however, a presidential candidate picks a running-mate who reinforces his perceived strengths.
Thus, Bill Clinton in 1992 chose Al Gore, who - like Mr Clinton - was a youthful centrist from a southern state.
If Mr McCain wishes to pursue the first strategy, and pick a running-mate who offers voters things that he himself cannot, then he has a number of options:
- The most obvious limitation that Mr McCain may want to address is his age: if he wins, he would be the oldest president ever elected to a first term, so he may want to appoint a more youthful running-mate
- For some in the party, Mr McCain is just not conservative enough, so if he wishes to reassure his party's base he may decide to pick a more right-wing running-mate
- Since Mr McCain will be facing either an African-American or a woman in the general election, he may opt to run with a black or female colleague on his ticket
- In order to broaden his geographical appeal, the Arizona senator may wish to go for a running-mate from the south or the mid-west
When making his decision, he may also consider picking someone who could help to deliver him a key swing-state in November
So does any one candidate meet all of these requirements?
Not quite - but a number of possible running-mates do tick a lot of the right boxes.
Popular Florida Governor Charlie Crist may well have tipped the balance for Mr McCain when he endorsed him ahead of his state's crucial primary earlier this year.
He would be young enough - and conservative enough - to complement Mr McCain, and he could help to ensure that the swing state of Florida stays in the Republicans' column in November.
Other young, popular, southern governors who could fulfil a similar function for Mr McCain would be Haley Barbour of Mississippi or Mark Sanford of South Carolina, although both states are very safe territory for the Republicans, so choosing Mr Barbour or Mr Sanford would not add a new state to the Republicans' tally in the general election.
Ms Rice could counter a potentially ground-breaking Democrat
Minnesota, however, could be a key battleground, and the state's well-liked governor, Tim Pawlenty, has been talked about as a possible running-mate for Mr McCain ever since Mr Pawlenty endorsed the Arizona senator back in 2006.
Like Governors Crist, Sanford and Barbour, Mr Pawlenty is more conservative than Mr McCain, and at only 47, he also has youth on his side.
If Mr McCain wishes to match the potentially historic nature of the Democrats' candidate this year, then choosing a young, experienced African-American woman like US Secretary of State Condoleezza Rice might help.
But if Ms Rice proved to be too moderate for Republican party activists, then another potential female running-mate could be Texas Senator Kay Bailey Hutchison, although she would be less likely than the moderate Ms Rice to deliver any new states for Mr McCain in November.
So there are a number of candidates for a McCain running-mate who balance out his perceived negative attributes.
What of the candidates who could reinforce Mr McCain's political qualities?
Gen Petraeus's political positions are not widely known
If Mr McCain wanted to stress his national security credentials and his support for the "surge" in Iraq, then he might pick someone like Gen David Petraeus, who has been in charge of US forces in Iraq since the surge began.
Little about Gen Petraeus's views on non-military matters is known, however, so Mr McCain may shy away from appointing him to such a high-profile position.
And whether Gen Petraeus would even accept an offer from Mr McCain is not clear.
Another left-field choice might be Connecticut Senator Joe Lieberman, who was Democratic presidential candidate Al Gore's running-mate in 2000, but later became an independent.
In December he endorsed Mr McCain ahead of all of the Democratic presidential candidates.
Although he is respected by Republicans for his conspicuous support of the Iraq War, his views on certain social and economic issues would probably be too liberal for many party activists to stomach.
Despite media speculation, Mr McCain is unlikely to add his primary opponent Mike Huckabee to his ticket.
Although Mr Huckabee could broaden Mr McCain's appeal among evangelical voters, he would be as unpopular as Mr McCain himself is with influential conservative commentators, who are critical of Mr Huckabee's populist stance on government spending.
Mr McCain's other main primary rival - Mitt Romney - would be popular with fiscal conservatives and social conservatives, but Mr McCain is thought to have developed a dislike for the former Massachusetts governor during the campaign.
Whoever Mr McCain picks to be his running-mate, the eventual choice is likely to have two crucial attributes.
Mr McCain will want to have a personal rapport with his chosen candidate.
And he or she must possess - in the eyes of the voters and the media - the ability to appear "presidential".
Because if Mr McCain wins, his running-mate may be called upon at a moment's notice to fulfil the most important function of any vice-president: to take over the top job in the event of a tragedy.