Barack Obama says he's picked a running mate, and that his selection had to meet three standards to join the Democratic ticket: be prepared to be president, help him govern and challenge his thinking.

Meanwhile, the presumptive Democratic nominee continued to keep everyone waiting Friday, repeating his plan to disclose the selection through text messages to supporters.

"Obviously, the most important question is: Is this person ready to be president?" Obama said in an interview aired Friday on CBS. Second, he said: "Can this person help me govern? Are they going to be an effective partner in creating the kind of economic opportunity here at home and guiding us through some dangerous waters internationally?"

He added: "I want somebody who is going to be able to challenge my thinking and not simply be a 'yes person' when it comes to policymaking."

On the Republican side, several GOP officials said Friday that John McCain had not settled on a running mate, though sources said former Massachusetts Gov. Mitt Romney and Minnesota Gov. Tim Pawlenty were under serious consideration.

McCain campaign officials said they were preparing for an "unconventional" nominee, an indication that oft-mentioned former Pennsylvania Gov. Tom Ridge, an abortion-rights supporter, or Connecticut Democrat-turned-independent Joe Lieberman still could be in the running. That category also could include non-politicians who McCain deeply admires, such as Gen. David Petraeus, the top U.S. commander in Iraq.

Two officials close to Romney said he had not been offered the job. Pawlenty batted away questions Friday in a CNN interview, saying, "I'm sure he'll make a wonderful choice for our party and for our country and we'll just have to wait until next Friday to find out the answer to those questions."

The Arizona Republican is expected to announce his choice between the Democratic National Convention that begins Monday and his own GOP coming out party in St. Paul, Minn., that begins Sept. 1. It's possible he could do it on Friday, Aug. 29, the day he turns 72 and the day after Obama accepts his own nomination in Denver, but no plans are set and he could wait until his own convention week.

Republican and Democratic officials said both candidates were capable of making wild card picks that would surprise their backers.

Little-known Texas congressman Chet Edwards is emerging as a finalist for Obama, according to Democratic officials who say that Edwards was one of the few Democrats whose background was checked by Obama's campaign.

And, although it appeared unlikely, Obama's vanquished rival Hillary Rodham Clinton still could emerge as his No. 2.

The Illinois senator was widely thought to be considering Virginia Gov. Tim Kaine, Gov. Kathleen Sebelius of Kansas and Sens. Joe Biden of Delaware and Evan Bayh of Indiana. None of them gave anything away -- at least not in words.

"I've made the selection, that's all you're gonna get," Obama told The Associated Press.

Kaine plans to fly Friday night directly from Virginia to Denver, site of next week's convention, three people with knowledge of the governor's travel plans said. The plans could be changed if Kaine is told he needs to fly to Springfield, Ill., for a planned Saturday Obama rally instead.

Biden had a family gathering at his home Thursday afternoon, with his wife Jill, niece Missy Owens and son Beau, Delaware's attorney general, coming and going past reporters staked outside.

Sebelius, campaigning for Obama in Iowa, said she would leave the announcement to the campaign.

Bayh worked in his Capitol Hill office and later spent time at his home in Washington. He left wearing shorts and a baseball cap but told reporters outside he had no news to share. "Not tonight, sorry," he said.

Two long-shots appeared out of the running. Former Georgia Sen. Sam Nunn's spokesman said he would be traveling internationally until Monday, making a Saturday visit to Illinois seem unlikely. And, Sen. Jack Reed of Rhode Island, a national security expert, told an AP reporter that he was not Obama's choice and that he had not been asked for any background information.

FOX News' Aaron Bruns and The Associated Press contributed to this report.