The next major milestone in Barack Obama's presidential campaign, following his Berlin moment in front of 200,000 ecstatic Germans during his global tour, will be his selection of a running mate.

The selection of a vice-presidential candidate is "the one last remaining wild card in any campaign for the presidency," Senator Obama's spokesman, Robert Gibbs, said outside Downing Street as the candidate wrapped up his 16,000-mile tour in London. "It will cause a pretty big ripple," Mr Gibbs predicted.

Expectations quickened last week that Mr Obama's rival, Senator John McCain, was preparing to announce his running mate in an attempt to upstage the Democratic contender's keynote speech in Berlin last Thursday. Mr Obama's speech, in which he urged a renewal of US ties with Europe as part of his agenda for change, was the centrepiece of the tour that took him to Afghanistan, Kuwait, Iraq, Israel, the Palestinian capital Ramallah, Germany, France and Britain. Foreign leaders of all political stripes fell over themselves to meet him in hopes of catching a little stardust from the charismatic American politician, who is often described as a new JFK.

But last week, instead of making the momentous announcement of a running mate, Mr McCain was out campaigning in the Mid-west, sniping at Mr Obama who on Saturday was forced to explain why he had dropped a planned visit to an American military hospital in Germany. A McCain advert aired over the weekend said: "he made time to go to the gym, but cancelled a visit with wounded troops".

The Illinois Senator responded that he had pulled out of the visit because the Pentagon had advised that it could be considered as political if he took along an adviser, retired Major-General Scott Gration.

The Democratic candidate also addressed concerns that his focus on foreign affairs might backfire at a time when American voters are struggling to meet mortgage repayments and facing unprecedented rises in the price of petrol and food.

He acknowledged in comments to reporters after two hours of talks with Gordon Brown that there may be a dip in his popularity ratings as a result of his weeklong absence. "People are worried about gas prices. They're worried about home foreclosures," he said. But he also argued that foreign policy and domestic issues are intrinsically linked, with the conflicts in Afghanistan and Iraq a huge drain on US resources.

Mystery continues to surround the prospective running mates, although speculation has intensified as the field narrows. The Democratic search committee, led by Caroline Kennedy and Eric Holder, has floated the name of a Republican former agriculture secretary, Ann Veneman.

Unconfirmed reports that the former presidential candidate John Edwards had fathered a child with a filmmaker while his wife is battling cancer, could doom his prospects. Other names that could be in the frame are those of his fellow senators Chuck Hagel, who accompanied Mr Obama to Iraq, Joe Biden and Chris Dodd, and the retired Marine General James Jones. As well as two governors, Virginia's Tim Kaine and Kathleen Sebelius of Kansas.

On the Republican side, Mr McCain has been in talks with Mitt Romney, his former rival for the presidency, Minnesota's Governor Tim Pawlenty, with whom he has a strong friendship; and the former House Representative Rob Portman of the battleground state of Ohio. But, given that both presidential candidates are serving senators, it could be that they would look outside Congress for a running mate. Carly Fiorina, a former CEO of Hewlett-Packard, is being mentioned as a partner for Mr McCain.

Initial signs were that Mr Obama's week-long trip had done him no harm at home. Opinion polls on Saturday released by Gallup found that he now led Mr McCain nationwide by 48 per cent to 41 per cent.