CHICAGO - Democrat Barack Obama stoked furious speculation by holding fire on his running mate announcement Friday, as a Texan dark horse candidate deepened the mystery around the closely guarded decision.

The Illinois senator's campaign meanwhile hurled new blows against his multiple-home-owning Republican rival John McCain, branding him aloof from US economic woes, in the run-up to next week's Democratic convention.

Obama says he has already selected his vice presidential running mate and according to aides was holed up in a downtown Chicago hotel to work on the convention acceptance speech he will deliver in a Denver stadium next Thursday.

The campaign has promised to release the VP news first to signed-up supporters via cellphone text messages and email. But TV networks were taking no chances, camping outside the homes of the potential candidates and at Chicago's Midway airport.

Obama's choice will be unveiled at the latest at a campaign event Saturday in Springfield, Illinois, the town where he first launched his White House bid in February 2007.

A little-known dark horse entered the pack as Texan lawmaker Chet Edwards, whose House of Representatives district encompasses President George W. Bush's Crawford ranch, disclosed that he had been vetted by Obama aides.

"I've been considered throughout this process," he told CNN, welcoming the support he has received as a prospective VP by House Speaker Nancy Pelosi, the country's top elected Democrat.

"But I've respected the process from day one and I want to continue to do that today, and allow any details about the process and final decisions to be made by Senator Obama and their campaign, not by anybody else," Edwards said.

One politician apparently ruling herself out was Hillary Clinton, whom Obama vanquished for the Democratic nomination and who is now being watched attentively for any signs of unhappiness as she prepares to speak in Denver.

"I am not in that arena. This is his decision and I respect him to make it however he thinks is best for him and the country," the New York senator and former first lady told reporters.

Clinton, who has campaigned this week in Florida on Obama's behalf, denied that she had been less than wholehearted in her support of the new Democratic champion.

"I think it's a fair assessment that I've done more than anybody else in my position," she said.

In a CBS interview that aired early Friday, Obama again refused to drop any VP hints and dwelled instead on attacking McCain as being "out of touch" with the anxieties of ordinary voters.

The Democrat said he himself had only struck it rich in the last couple of years, as his book sales took off.

"John McCain's been living like this for the last 25. And, obviously, doesn't have a very clear sense of what ordinary Americans are going through," he said.

McCain was also off the campaign trail Friday, finalizing his own VP pick days before next week's Democratic National Convention marks the formal start of hostilities for November's presidential election.

While both the candidates will be officially anointed their party nominees over the next fortnight, they have been locked in running battles for weeks already -- and are now engaged in some of the fiercest clashes yet.

The Obama camp has since Thursday been heaping scorn on McCain after the Arizona senator, in an interview with Politico.com, could not say how many homes he and his wealthy wife own.

The Democrat's second ad on the gaffe said the number was seven, although USA Today said it could be as high as 12, and noted his jokey assertion that only an annual income of five million dollars qualifies someone as rich.

"Maybe McCain thinks this economy is working -- for folks like him," the ad's sorrowful narrator says. "But how are things going for you?"

McCain's campaign battled to limit the damage by pointing to Obama's ties to convicted fraudster Tony Rezko, a Chicago businessman and fundraiser who helped the Democrat with the purchase of his spacious family home.