DENVER — A cancer-fighting Sen. Edward M. Kennedy prepared to attend, and possibly speak, at the opening day of the Democratic National Convention today as presidential nominee-to-be Barack Obama unleashed a hard-hitting television commercial linking GOP rival John McCain with President Bush.

The ad signaled that the Democratic gathering would be just as much about skewering McCain as it would be about unifying the Democratic Party following a protracted primary season that split the party between Obama and Sen. Hillary Rodham Clinton of New York.

Kennedy, who is being treated for a malignant brain tumor, is a beloved figure within the Democratic Party, and the Massachusetts senator’s last-minute appearance at the Pepsi Center is a way to unite the fractured party as the four-day convention opens amid signs of acrimony between Obama and Clinton delegates.

Kennedy arrived in Denver Sunday night and got a checkup at a local hospital. He plans to attend to watch a video tribute to him and may even address the convention if he feels up to it, said a senior Democratic official who talked on the condition of anonymity.

“He’s truly humbled by the outpouring of support and wouldn’t miss it for anything in the world,” said Stephanie Cutter, a Kennedy spokeswoman.

As Democrats put the final touches on opening night, Obama’s campaign released an ad featuring images of McCain hugging Bush and the two smiling in spite of tidings of economic woe. It features a parody of the Sam Cooke classic “Wonderful World,” which starts off with the line “Don’t know much about history.” For the ad it’s “I’m not up on the economy,” playing on McCain’s earlier admission that economics wasn’t his best subject.

Ending with a photo of Bush patting McCain’s back, the spot asks, “Do we really want four more years of the same old tune?”

McCain’s campaign also released an ad to play on what it sees as a weakness for Obama: his lack of support among some Clinton backers. Its new TV ad features a Clinton supporter who now backs McCain assuring like-minded voters: “A lot of Democrats will vote McCain. It’s OK, really!”

Opening night at the Pepsi Center, the main venue for the four-day convention, aimed to tell the Illinois senator’s personal story to the millions of voters nationwide who will begin tuning in to the presidential campaign. Obama’s wife, Michelle, was the evening’s keynote speaker.

Obama’s campaign dismissed concerns about the impact of die-hard Clinton supporters on the choreographed show of unity. Behind the scenes, however, polls showing significant Clinton support still being denied to Obama and pro-Clinton demonstrations at offsite venues were creating a different kind of anticipation. Clinton has backed Obama and was scheduled to speak Tuesday night.

“There are a lot of delegates here who had passionate choices in an extended primary season,” Obama spokesman Robert Gibbs told “Today” on NBC. “We feel confident that if we can demonstrate a record of change, a record of vision … a team of Barack Obama and Joe Biden can convince Democrats, Republicans and independents to support a ticket of change in November.”

Most Democratic delegates were putting the rough-and-tumble primary contest behind them and focusing on electing the first black presidential nominee of a major political party. The night was turned over to Michelle Obama, the candidate’s wife of nearly 16 years, to allow the potential first lady a prime-time speech meant to serve a dual purpose: humanize Obama and show up her own critics before her largest audience yet.

“Our stories are the quintessential American stories,” Michelle Obama said in an interview CNN aired today. “I am here because of the opportunities that my father had, that my mother had. You know, we are who Americans were supposed to be.”

With Democrats and convention delegates streaming to the Mile High City, party officials worked to assure a harmonious week.

Clinton was expected by midweek to release the delegates she won in primaries and caucuses and encourage them to support her former rival.

And by unanimous vote, the party’s credentials committee restored full voting rights to delegates from Florida and Michigan. The party had stripped both states of their voting rights for holding primaries before the rules said they could. The committee vote was taken at Obama’s behest, and Democrats hope the goodwill gesture will help improve their standing in two important states.

Obama, slowly making his way to Denver via a tour of battleground states, said Sunday that one of his goals is for voters to come away from the convention thinking he is one of them. His uncommon name and family background still concern some voters.

“I think what you’ll conclude is, ‘He’s sort of like us,”’ Obama said in Eau Claire, Wis. “‘He comes from a middle-class background. He went to school on scholarships. He had to pay off student loans. He and his wife had to worry about child care. They had to figure out how to start a college fund for their kids.”

Republicans seek to portray Obama as an Ivy-league-educated elitist. His background — white mother from Kansas, black father from Kenya, raised in Hawaii and Indonesia — has been grist for the rumor mill, leading some voters to believe he is a Muslim or unpatriotic.

Obama closes the convention Thursday night when the action shifts to Invesco Field at Mile High stadium, where the 47-year-old, first-term senator will give his speech accepting the nomination from the 50-yard line. He said Sunday he was “still tooling around with my speech a little bit.”

He is scheduled to campaign today in Iowa.

McCain, meanwhile, wasn’t disappearing from the campaign trail entirely. He was using an appearance today on “The Tonight Show with Jay Leno” and newspaper interviews to stay in touch with voters. And, there’s continued interest in his choice of a running mate.

Besides Michelle Obama, other speakers today night include Barack Obama’s sister, Maya Soetero-Ng, and Craig Robinson, his brother-in-law. The schedule includes a surprise speaker, former Rep. Jim Leach of Iowa, a Republican moderate who broke ranks with his party this month and endorsed Obama.