Speculation about vice presidential picks for Barack Obama and John McCain hit top gear Wednesday as the candidates revved up their political machines for the decisive phase of the White House race.

Senator Obama's choice of running mate was expected to be announced first in the run-up to next week's Democratic nominating convention in Denver. McCain faced heavy flak from the Republican right for airing the possibility of a fielding a pick who might favor abortion rights.

Commenting on the heated rumor mill, Obama said at a farmers' market in Greensboro, North Carolina early Wednesday: "No hints. No new hints."

Before swinging north to Virginia, Obama told a rally in North Carolina late Tuesday that his running mate would never be mistaken for the current vice president, the secretive and enormously influential Dick Cheney.

The media buzz was ringing loudest around Virginia Governor Tim Kaine, Indiana Senator Evan Bayh and Delaware Senator Joseph Biden.

If it does not come before, the choice will be unveiled in public Saturday at an event in Springfield, Illinois, the hometown of venerated Civil War president Abraham Lincoln where Obama launched his long-odds White House bid way back in February 2007.

Playing on frayed reporters' nerves, the Democrat's campaign early Wednesday emailed a press release entitled "Vice Presidential...." The next line said: "Just kidding."

Without naming any names, Obama fired a stinging critique of Cheney in arguing that his VP pick would not rewrite energy policy or foreign policy and would not hold himself above the law.

He repeatedly used the pronoun "he" when Obama, before, has been studiously gender-neutral in discussing his choice of running mate. That might definitively rule out his primary challenger Hillary Clinton.

"I want somebody who's independent, somebody who's able to tell me 'you know Mr. President, I think you're wrong on this and here's why'," the Illinois senator said at late Tuesday's rally in Raleigh.

Above all, he said, "I want somebody who is capable of being president, who I trust."

Biden, who would bring national-security heft to the 47-year-old Obama's ticket, was coy when cornered by reporters on his golf cart in Delaware Tuesday.

"I promise you, I don't know anything. I have no idea. I haven't spoken to no one," said the veteran chairman of the Senate's foreign relations committee.

Asked if he was ruling out joining the Obama ticket, Biden added: "I have no idea. You guys know as well as I do."

According to an analysis on the polling website fivethirtyeight.com, Biden, 65, polls particularly well among seniors -- a key demographic given their size and zeal to vote, and which favors McCain over Obama in all surveys.

But choosing the Pennsylvania native might also play up Obama's own relative inexperience, and would run counter to his central promise that he will revolutionize Washington's culture of insider politics.

Kaine would be the right kind of Washington outsider and his state of Virginia is pivotal to Obama's prospects for victory in the November 4 election. But he also lacks a national profile.

As Obama headed to his state, Kaine was evasive when pressed on the speculation. "The campaign is in control of all of the announcements and I'm not going to talk about my conversations with the campaign," he said.

Whatever his selection, Obama is in fighting mood heading into the convention season, which marks the countdown to a furious two months of coast-to-coast campaigning for November's election.

"I'm a big believer in winning. I don't intend to lose this election. John McCain doesn't know what he's up against right now," he said in Raleigh, as his campaign intensified a barrage of nationwide negative attack ads on McCain.

The Arizona senator, who turns 72 on August 29, meanwhile is expected to announce his own choice of VP nominee just before the week-long Republican convention starting on September 1.

One name apparently floated by the campaign is former national security chief Tom Ridge, but his liberal views on abortion have drawn intense fire from right-wing radio hosts and commentators.

Others in the frame include Minnesota Governor Tim Pawlenty and former Massachusetts governor Mitt Romney, who has been effusive in his praise of McCain after bitterly fighting him for the Republican nomination.