As strains between Obama and some former Hillary Clinton supporters persisted, Republicans brought out a new ad pointedly invoking her past criticism that Obama wasn't ready to lead.
The Democrats began two days during which their national convention will be dominated by the old Clinton regime.
Obama's former rival was expected to urge her disappointed supporters to line up in unity behind him in a prime-time speech Tuesday night. Her husband, former President Clinton, speaks Wednesday night.
McCain's latest TV ad played off her primary campaign spot featuring sleeping children and a 3 a.m. phone call portending a crisis. In the new ad Clinton is shown saying: "I know Sen. McCain has a lifetime of experience that he will bring to the White House. And, Sen. Obama has a speech he gave in 2002."
A narrator adds: "Hillary's right. John McCain for president."
Some Democratic activists, meanwhile, voiced concern that the convention, which began on Monday, had yet to produce a sustained or effective attack against Republican presidential candidate McCain.
In particular, they cited comments by former Virginia Gov. Mark Warner, who was Tuesday night's keynote speaker, suggesting that his speech would not be a red-meat attack on McCain but an appeal for bipartisanship.
"There may be parts of the speech that aren't going to get a lot of applause, but I've got to say what I believe will get our country back on the right path," Warner - who was neutral in the party primaries - told reporters on Monday.
Democratic strategist Paul Begala took issue with Warner's comments, suggesting that more partisanship, not less, was needed at the party convention.
"This isn't the Richmond Chamber of Commerce," Begala said Tuesday.
On Monday, James Carville told CNN: "If this party has a message, it's done a hell of a job hiding it tonight, I promise you that."
Both Begala and Carville were top strategists behind Bill Clinton's successful 1992 presidential bid.
Party chief Howard Dean took issue Tuesday with such remarks by fellow Democrats, as well as criticism that the party isn't doing enough to bring die-hard Clinton supporters into the fold.
"We don't need to attack McCain" during the convention's opening events, Dean told delegates from Ohio, a battleground state. "There will be plenty of time for that."
It's more important, he said, "to make sure people know who Barack Obama is, who Joe Biden is."
"There is not a unity problem," he added. If anyone doubts that, he said, "wait 'til you see Hillary Clinton's speech tonight."
Even so, internal strains remained among Democrats. Former party chairman Don Fowler, a former Clinton supporter, said he is uneasy about the attitudes of some Clinton delegates.
"All you need is 200 people in that crowd to boo and stuff like that and it will be replayed 900 times. And that's not what you want out of this," he said in an interview.
"This whole thing about the nomination roll call (for Clinton) ...to introduce that kind of uncertainty is just what you don't want."
Anna Burger, the chair of Change to Win, made up of seven unions, said some Clinton supporters were having a hard time letting go and switching loyalties to Obama. But, she said in an interview with The Associated Press, "the vast majority of them have."
A convention speaker Tuesday night, Burger said she hoped the remaining holdouts would follow suit. "We have to leave here Friday ready for action," she said.
Meanwhile, McCain is expected to name his running mate in the coming days.
A top surrogate and prospective No. 2, former Massachusetts Gov. Mitt Romney was to be in Denver on Tuesday to assail Democrats on behalf of McCain. It amounted to a final audition of sorts as McCain seeks a strong running mate who can play the attack-dog role against Obama and running mate Joe Biden. Another Republican said to be a serious vice presidential contender, Minnesota Gov. Tim Pawlenty, was to be in Denver doing the same on Thursday.
Biden, in his first public remarks since being named Obama's running mate, told home-state Delaware delegates he "didn't always comport myself in the way that I wanted to."
He did not elaborate, but aides said it was mostly a reference to Biden's reputation for long-windedness and off-the-cuff remarks that sometimes backfired. He ended his 1988 presidential run amid allegations of plagiarism. As he began a 2007 run, he called Obama "articulate" and "clean." He also drew criticism for saying "you cannot go to a 7-Eleven or a Dunkin' Donuts unless you have a slight Indian accent."
Biden, at times emotional, thanked fellow Delaware Democrats for their tolerance as his large family listened.
Clinton once seemed to have the nomination in her grasp and now is being called on to defend and support the person who wrested it from her. She is effectively playing middlewoman Tuesday night - passing a torch from her husband, the 42nd president, to Obama, who wants to succeed him as the next Democratic president.
But not without some Clinton-style political dealmaking and drama.
The Clinton and Obama camps agreed to limit Wednesday's potentially divisive nominating process for president, allowing some states to cast votes for both Obama and Clinton before ending the roll call in an acclamation for the Illinois senator.
In one scenario, Clinton herself would cut off the voting and urge the unanimous nomination of Obama, according to Democratic officials involved in the negotiations.
But some Clinton delegates said they were not interested in a compromise, raising the prospect of unwelcome floor demonstrations.
Gloria Allred, a California celebrity lawyer and pledged Clinton delegate, briefly disrupted a breakfast meeting of the California delegation on Tuesday. Wearing a gag over her mouth, she protested efforts to discourage Clinton supporters from speaking out.
"There is no doubt in anyone's mind that this is Barack Obama's convention," Clinton said on Monday. And yet, she said, some of her delegates "feel an obligation to the people who sent them here" and would vote for her.
As part of the deal, Obama and Clinton activists teamed up and circulated three petitions on the convention floor Monday night - supporting submission of Clinton's and Obama's names for president in the roll call and Biden's for vice president. Each needed 300 signatures.
As well as the speeches by Clinton and Warner, the lineup for the second day of the convention features 11 governors and prominent House and Senate leaders.
Former Vice President Al Gore, Virginia Gov. Tim Kaine and Illinois Sen. Dick Durbin are among the speakers lined up for Thursday night's convention finale.
Obama will accept the nomination before an estimated 75,000 people at the Denver Broncos' football stadium that night.
Associated Press writers Jim Kuhnhenn, Liz Sidoti, Nedra Pickler, Charles Babington and Stephen Ohlemacher contributed to this story.
(Copyright 2008 by The Associated Press. All Rights Reserved.)