U.S. Republican presidential hopeful John McCain's camp released its second ad since Democrat Barack Obama announced Sen. Joe Biden for his choice as running mate, questioning why Sen. Hillary Rodham Clinton wasn't picked.

The latest ad questions Obama's motives in passing over Clinton, his chief rival for the nomination, and deciding on Biden, who dropped out of the race after the first contest in Iowa. 

Released early Sunday, the ad features clips of Clinton during the primary battle criticizing Obama. "Senator Obama's campaign has become increasingly negative," it had her saying.

An announcer said: "She won millions of votes but isn't on the ticket. Why? For speaking the truth."

Chief Obama strategist David Axelrod, who appeared on ABC's "This Week," said the senator has a "high regard" for Clinton, who will be an "important voice in moving this country forward in the next administration. But he felt Senator Biden would be the best fit for him at this time."

Caroline Kennedy, a member of Obama's search team, said "Joe Biden was absolutely the best" choice, adding that Clinton is an admired figure who Obama will need in Senate.

House Speaker Nancy Pelosi, D-Calif., said Biden has "the full package," and is someone who has challenged the status quo.

"He's even criticized Senator Obama, so it's a tribute to Senator Obama that he's not just choosing a yes man but a person who will speak what he believes," she said.

Biden is chairman of the Senate Foreign Relations Committee.

Potential swing-state governors Tim Kaine of Virginia and Bill Ritter of Colorado, who appeared on "Fox News Sunday," also supported Biden's selection.

Kaine, who had been on Obama's short list for a running mate, said Obama and Biden were "a good team personality-wise. They complement each other well. I think you're going to see them really enjoying being out on the trail together."

Ritter said Obama and Biden would help the ticket, particularly with independent voters in the West.

McCain, an Arizona senator who has yet to announce his running mate, told CBS News Sunday that Biden was a "wise selection." But McCain was critical of Biden's decision to vote against the first Gulf War and the notion that Iraq should be divided "into three different countries."

Biden returned to Delaware after the first joint appearance Saturday in Springfield, Ill., where Obama began his campaign in early 2007.

Obama resumed campaigning with a scheduled stop in Wisconsin in the run-up to accepting the Democratic nomination for president in Denver Thursday.

Obama's challenge

Dennis Goldford, professor of political science at Drake University in Des Moines, Iowa said Obama's challenge will be to connect with the individual during his speech.
 
"While he tends to fly high in the clouds when he addresses people, which is part of his charm for many people, he's got to be able to make people watching believe he is talking to each one of them individually," Goldford told CTVNewsnet. "He's got to establish that one-on-one connection because that's what he's had difficulty doing so far."

Sen. Clinton played on the lack of connection, and Sen. McCain will do the same. Therefore voters need to feel they have a sense of Obama, even if it's through television or radio, Goldford said.

The convention will fall on the 45th anniversary of the Rev. Martin Luther King's legendary "I have a dream" speech.
 
Goldford said Obama followed King's notion of "people of different backgrounds being able to live together harmoniously" in his 2004 speech, disregarding red or blue and calling for a united States of America.

Obama will most likely continue in the same vein but talk in more concrete terms about what his presidency would mean to the everyday person, he said.

Clinton is slated to speak at the convention, which will be important because she still has a large number of supporters, he said. But if she and former President Bill Clinton don't make an honest effort to unite the Democrats, it will reflect poorly on them and cost future party support.

With files from The Associated Press