Barack Obama is poised to announce his choice of running mate ahead of a massive rally in Illinois tomorrow where he will appear with his vice-presidential pick.

The Democratic nominee spent today at home in Chicago, telephoning shortlisted candidates whom he had not chosen, as journalists camped out on the front lawns of several contenders looking for clues into one of the closest held secrets of this year's presidential campaign.

The rumours and second guessing were intense, with attention focusing on three men in particular: Joe Biden, the veteran senator and foreign policy heavyweight; Evan Bayh, a centrist fellow senator from Indiana; and Tim Kaine, the Virginia governor.

Yet after the total surprise picks of Dick Cheney in 2000, and Dan Quayle in 1988, many were also bracing themselves for an unexpected choice. One Democratic source said that Hillary Clinton had never been vetted - and had not been asked to turn over one document - but there was still speculation that Mr Obama could yet turn to the former First Lady.

One name that emerged yesterday was Chet Edwards, a little known Democratic conservative congressman who represents the Texas district where President Bush owns his ranch. Aides confirmed he was fully vetted.

In an interview, Mr Obama said he had made his choice, adding that he had picked someone he believes could step in to be president, help him govern and somebody who could challenge his thinking - not a "yes person". The choice will first be e-mailed to Mr Obama's supporters.

With the Democratic convention opening in Denver on Monday, Mr Obama will travel to battleground states with his running mate before arriving in Colorado next Wednesday. He will accept the nomination before a crowd of 75,000 in Denver's Invesco football stadium on Thursday night - and not in the convention centre that will host the first three days - a move John McCain's campaign says smacks of arrogance.

Mr McCain, meanwhile, is also poised to choose a running mate, but is waiting to see whom Mr Obama picks before making a final decision. Two men are receiving serious attention: Mitt Romney, the former Massachusetts governor and presidential rival, and Tim Pawlenty, the Minnesota governor.

The Republican is considering announcing his choice in the critical battleground state of Ohio on Friday, August 29 - his 72nd birthday, the day after Mr Obama's acceptance speech and three days before his nominating convention opens in Minneapolis.

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