By Richard Lister
BBC News, Washington
Now that he is the Republican presumptive nominee, there is one central question facing John McCain.
John McCain has a little breathing space to work out his strategy
Who is he competing against?
Until the knock-down, drag-out, everything-to-play-for Democratic party nomination contest is resolved, the Arizona senator does not know quite how to finesse his campaign.
Will he be taking on the first female presidential candidate in American history; a pillar of the Democratic party machine with a comprehensive package of policies, a huge contacts book, support in the big states and a presidential husband with both baggage and strategic brilliance?
Or will he be up against America's first black candidate; someone who came from behind, riding a wave of euphoria generated by his soaring oratory and rock-star appeal?
In that case, he faces a candidate with limited experience of high level politics, who always opposed the Iraq war, brings out the youth vote, appeals (like Mr McCain does) to independents and seems to have made few real mis-steps in his political career that could be exploited.
Age and experience versus youth and inspiration? Or will it be a battle for the female vote?
What's a candidate to do? Well to some extent now Mr McCain has some breathing space to work it out.
Barack Obama and Hillary Clinton will continue to fight each other
There is time to recharge his own batteries - he is in his 70s after all - and re-fill his campaign war chest.
The money issue is one he needs to address. His opponents have been able to comprehensively out-raise him since the very start of this campaign and big spending on television ads and travel can swing close elections.
Mrs Clinton and Mr Obama will be spending fortunes trying to beat each other in coming weeks, but they have energised their party like never before.
Whoever wins the nomination will be able to tap into that Democratic excitement.
There is less Republican excitement about Mr McCain.
With the spotlight off him for now, he has some space to privately mend fences with big conservative donors who still wish someone slightly more, well, right-wing, had won the nomination.
At the same time, though, he doesn't want to drive away those independents who like him because of his more moderate record.
As the Democratic campaign continues, he has the chance to see which messages are resonating and where the floating votes are to be found.
But, and here's the thing, he cannot afford to give the stage entirely to Mrs Clinton and Mr Obama, and become the asterisk candidate - a short final paragraph in all the news stories about the Great Democratic Presidential Race.
Did Mike Huckabee's long presence in the race help Mr McCain?
One school of thought was that it helped him to have Mike Huckabee running a campaign against him.
Even though Mr McCain was the presumptive nominee for weeks, it was at least still a contest the news media felt obliged to cover.
Now Mr Huckabee has withdrawn, there is not the same impetus.
So, fairly soon, he has to come up with a campaign that makes him a distinctive candidate in what is, at the moment, a field of three.
There are signs that he has already started to do that - particularly on Iraq where he has clear differences with both his rivals.
He was bullish about the war in his Texas victory speech.
He said he stood up for his "decision to destroy Saddam Hussein's regime" (something neither of his opponents can do) and now that American losses there are out of the headlines, it is an approach that may bolster his image as a "national security candidate".
Of course, the situation in Iraq could change again, and he could find himself again on the wrong side of an unpopular war.
Hard to beat?
There is another potential Achilles heel too.
George W Bush's endorsement could prove a double-edged sword
After clinching the nomination, his first stop was the White House to pick up the formal endorsement of President George W Bush.
That is a double-edged sword, if ever there was one.
Mr Bush may certainly be able to reach into some deep donor pockets, but his approval ratings have hit near-record lows and Mr McCain has to find a way of avoiding being cast as the "more-of-the-same" candidate.
He can, though, take comfort from one thing.
Despite all the excitement, money and coverage generated by the Democratic campaign, the opinion polls still suggest that Mr McCain will be extremely hard to beat in a general election.
He must be doing something right.