Democrats opened their convention Monday to anoint Barack Obama as the party's presidential candidate, setting in motion a four-day political festival that seeks to heal wounds opened in a bruising primary battle and reassure voters still wary of putting a political newcomer in the White House. Obama needs a jolt to his campaign after Republican rival John McCain wiped out his lead in the polls despite a widespread desire for change in an era of economic uncertainty, continuing conflict in Iraq and poor approval ratings for Republican President George W. Bush. The party sought to use opening night to reintroduce Obama, with an emotion-filled appeal to the party's rank and file by one of the party's beloved figures, the ailing Sen. Edward M. Kennedy. The appearance of Kennedy, the scion of a liberal political dynasty, had been in doubt because he is being treated for a malignant brain tumor. But, in a strong voice, he said, "I pledge to you that I will be there next January on the floor of the United States Senate," when the next president takes office. He urged the country to become committed to Obama's cause. "The work begins anew, the hope rises again and the dream lives on," he said, reprising the final line of a memorable 1980 speech, when Kennedy was running for president, that brought a different convention to its feet. Obama's wife, Michelle, was the evening's featured speaker, casting herself and husband Barack as people guided by bedrock American values. "We want our children - and all children in this nation - to know that the only limit to the height of your achievements is the reach of your dreams and your willingness to work for them," she said in excerpts released in advance of her appearance. Behind the scenes, Obama's representatives reached an agreement with former rival Hillary Rodham Clinton's camp to allow delegates to vote for either candidate in the roll call vote that will seal the nomination for Obama. Democrats wanted to avoid a divisive roll call while giving Clinton supporters a limited chance to express their support. Though Clinton has asked her supporters to back Obama, a new poll showed 30 percent of her backers would not. The leader in the House of Representatives, Speaker Nancy Pelosi, acknowledged that her fellow Democrats "had not yet achieved the complete reconciliation that we need." Obama stunned the US political establishment by defeating Clinton, the most powerful name in Democratic politics, during a drawn-out primary campaign. He shattered fundraising records as he ran on a message of hope and change. But Obama, a 47-year-old first-term senator, has struggled to win the support of white working-class voters, many of whom supported Clinton and could be pivotal in the November election. Some doubt that he has the experience to lead the nation. He has also had to fend off challenges to his patriotism and rumors that he is a Muslim. "There are people who are not going to vote for him because he's black," said James Hoffa, president of the Teamsters union, one of the nation's largest. "And we've got to hope that we can educate people to put aside their racism and to put their own interests No. 1." He spoke in an Associated Press interview. Obama also is battling attacks by McCain, who questions the relative newcomer's national security credentials and is critical of his opposition to offshore oil drilling at a time of record-high gasoline prices. Obama's campaign has become more aggressive, however, and on Monday unleashed a mocking ad that links McCain with Bush and failed economic policies. The musical commercial underscored a desire by party leaders to use the convention to skewer McCain even as it seeks unity in the Democratic family. Obama also announced this weekend his choice of Joe Biden, a veteran senator and aggressive campaigner ready to take on McCain despite their decades-long friendship. But the choice of Biden may have exacerbated tensions with Clinton supporters, many of whom had hoped she would become Obama's running mate. In remarks to the New York delegation to the Democratic convention here, Clinton declared too much was at stake for the party to remain divided. She cited the next president's hand in naming Supreme Court justices, the need to fix American education, the US energy crisis and foreign policy challenges. "None of that will happen if John McCain is in the White House. I just want to make it absolutely clear we cannot afford four more years of George W. Bush's failed policies in America, and that's what we would get with John McCain," she said. The efforts to heal divisions has added drama to a carefully staged event. US political party conventions once featured the possibility of numerous roll call votes for more than one presidential nominee and intense speculation about running mates. Now both nominees are known well before the opening gavel, and parties use their days in the spotlight to showcase their candidates. Obama will accept his nomination Thursday night in a speech to 75,000 in an outdoor sports stadium. He said Monday he would use the speech to help voters understand what he planned to do to improve the economy. "I'm not aiming for a lot of high rhetoric," Obama said. The Republicans hold their convention next week in St. Paul, Minnesota. McCain has begun a busy week of low-key events, and has tempered his rhetoric somewhat. On Monday, he called Obama his "very honorable opponent."