Aug. 25 (Bloomberg) -- Barack Obama and John McCain exchanged a stream of attacks as Democrats gathered in Denver to open their national convention, where Michelle Obama will take center stage tonight.

As delegates poured into the city for four days leading up to Obama's formal nomination Aug. 28, one of the few open questions was how the tension between Obama and Hillary Clinton supporters will play out.

McCain released a new advertisement titled ``Passed Over,'' suggesting that Clinton isn't on the ticket because Obama was stung by her criticism of him during the primary campaign.

David Axelrod, Obama's chief strategist, fired back by hammering that a McCain presidency would amount to a third term for the Bush administration.

``I know that the Republicans would like to shift the focus, but the real focus is whether we can endure more of these same policies,'' he said yesterday on ABC's ``This Week.''

The back and forth between the campaigns filled a void before the convention is gaveled to order with Michelle Obama booked as the headliner.

Her solo appearance tonight will be her first address to a broad audience of voters and it's a chance for her to help frame her husband's biography.

Family Affair

The prime-time speech will be a family affair. Marian Robinson, Michelle's mother, will narrate a video about the next potential first family, and Michelle will be introduced by her brother, Craig, head basketball coach at Oregon State University in Corvallis.

While the 15-minute speech will be peppered with anecdotes about the couple and their two daughters, Malia, 10, and Sasha, 7, it will mostly be a humanizing portrait of the candidate, who has endured criticism from Republicans, and even some Democrats, for appearing aloof and elitist.

``You will have a sense of who she is and what our values are and how we are raising our kids, and I think what you will conclude is `gee, he's sort of like us,''' Obama told voters yesterday in Eau Claire, Wisconsin.

The address also is a reintroduction for Michelle Obama, 44, who had been the object of criticism for some of her remarks during the primary.

Republicans seized on comments she made in February when she said her husband's candidacy made her feel proud of her country ``for the first time in my adult lifetime.'' She later said she meant to say she was proud that Obama's campaign had sparked enthusiasm. In April, the National Review magazine dubbed her ``Mrs. Grievance.''

Softer Image

After tapping veteran Democratic strategist Stephanie Cutter as her chief of staff in June, Michelle Obama has softened her image by appearing on television programs such as ``The View.'' Going into this week's convention, she is on the covers of Essence and Ebony magazines.

``Her first job on Monday night is to reintroduce herself to the country and to the world because she's never had a larger audience,'' said Myra Gutin, a communications professor at Rider University in Lawrenceville, New Jersey, and author of a book about first ladies. ``The second task is to introduce her husband to the audience in a way in which she knows him and perhaps we don't.''

Ivy League Credentials

Like her husband, Michelle Obama boasts Ivy League credentials, with an undergraduate degree from Princeton University in New Jersey and a law degree from Harvard University in Cambridge, Massachusetts. She grew up on the South Side of Chicago in a single-wage family with a stay-at-home mother and a shift-working father who suffered from multiple sclerosis.

Tonight she breaks ground as she auditions for the role of the first black first lady.

``There haven't been opportunities for black women to be revered in a way of being referred to as first lady of a city, a state or as a nation,'' said Wellington Webb, Denver's first black mayor. Such a step would be ``very significant given our history.''

The task this week for Barack Obama, 47, is to build on his wife's introduction and to unify the party following a sometimes bitter primary fight between him and Clinton.

Illustrating the bruised feelings that remain, Pennsylvania Governor Ed Rendell, a Clinton supporter, said during a forum yesterday sponsored by Harvard's Joan Shorenstein Center on the Press, Politics and Public Policy, that Obama, ``running for the most important job in the world,'' got a ``free pass'' from the media during the primaries.

McCain Ad

McCain, 71, sought to exploit any lingering resentment with his ``Passed Over'' ad, in which the announcer says, ``She won millions of votes, but isn't on his ticket. Why?''

Obama chief strategist David Axelrod and longtime Clinton adviser Maggie Williams issued a joint statement denying there was any tension between the two camps.

``Our teams are working closely to ensure a successful convention and will continue to do so,'' Obama adviser Anita Dunn said, reading the statement today on a conference call.

The Politico reported on its Web site, citing unidentified people, that disputes had arisen over the focus of former President Bill Clinton's speech to the convention and that some of Obama's partisans were accusing Clinton supporters of engaging in power plays.

To contact the reporter on this story: Julianna Goldman in Denver at jgoldman6@bloomberg.netCatherine Dodge in Denver at cdodge1@bloomberg.net.

Last Updated: August 25, 2008 11:36 EDT