US President Barack Obama, delivering one of the most crucial speeches of his young presidency, urged Congress to set aside partisan bickering and work with him to overhaul the US health care system. In a nationally televised speech Wednesday night before a joint session of Congress, Obama aimed to seize the momentum on an issue that has divided fellow Democrats, energized opposition Republicans and contributed to the decline in the president's once-soaring popularity. The speech was a political tour de force. To the public, he offered assurances that his plan would provide more security and more health care choices, while offering coverage to people who cannot currently afford it. To Republicans, he offered a hand to work together and pledged not to raise the government's deficit. Yet for Democrats who want him to be more assertive, he lashed out at opponents whom he accused of using scare tactics and lies to bring down the plan - and his presidency. "I will not waste time with those who have made the calculation that it's better politics to kill this plan than improve it," he said. Obama appealed to emotions, unveiling a letter from Edward Kennedy, the respected Democratic senator who died last month. In the letter, delivered posthumously, Kennedy expressed confidence that the overhaul would pass this year. Kennedy's widow, Vicki, was in the chamber's visitor's gallery next to first lady Michelle Obama. And Obama returned to the soaring political rhetoric that marked his presidential campaign. "We did not come to fear the future. We came here to shape it," he told lawmakers. It is unclear if Obama persuaded any Republicans. In keeping with tradition, most sat silently or offered polite applause during the speech. But in an unusual outburst, one Republican congressman, Joe Wilson of South Carolina, shouted out "You lie" when the president said illegal immigrants would not benefit from his proposals. The president paused briefly and smiled, but from her seat in the visitor's gallery, first lady Michelle Obama shook her head from side to side in disapproval of the interruption. Wilson later apologized for his "lack of civility." Health care has become the definitive issue for Obama, just nine months after he took office amid enormous expectations at home and abroad. His success or failure may determine whether he has the political clout to press ahead on issues like climate change, arms control and the Afghanistan war. It is also likely to shape next year's congressional elections. The United States is the world's only developed country without a universal program of health care coverage. As many as 50 million Americans lack health insurance. While many Americans are dissatisfied with the health care system, attempts to change it are politically explosive. Obama's plan would impose new regulations on insurers while requiring all Americans to get coverage. He says it would drive down prices, prevent insurers from dropping sick patients and ultimately strengthen the economy by curbing exorbitant health care costs. He said the changes he wants would cost about $900 billion over a decade, "less than we have spent on the Iraq and Afghanistan wars, and less than the tax cuts for the wealthiest few Americans" passed during the Bush administration. Republicans have overwhelmingly opposed the plan. They see it as a step toward a government takeover of health care and fear it will raise costs while driving down quality. "Replacing your family's current health care with government-run health care is not the answer," Rep. Charles Boustany, a heart surgeon, said in the Republican response to the speech. If the summer belonged to opponents of his health care overhaul, Obama is hoping to lay claim to the rest of the year - and close it by getting a bill on his desk. A CNN/Opinion Research Corp. snap poll of people interviewed before and after Wednesday night's speech indicated that the president shifted public opinion in his favor. After the speech, two-thirds said they supported Obama's health care proposals, compared with 53 percent in a survey days before the president spoke. That contrasted with an Associated Press-GfK poll released hours before the speech that showed many Americans had become disillusioned with Obama's handling of health care. It found that disapproval of Obama's handling of health care has jumped to 52 percent, from 43 percent in July. The White House has struggled to counter false rumors spread on conservative radio and the Internet. At one point in his speech, in a line apparently aimed at former vice presidential candidate Sarah Palin, Obama accused Republicans of spreading the "cynical and irresponsible" charge that the legislation would include "death panels" with the power to hasten the death of senior citizens. Criticizing Republicans without saying so, Obama said: "Instead of honest debate, we have seen scare tactics" and ideological warfare that offers no hope for compromise. "Well, the time for bickering is over," he said. "The time for games has passed. Now is the season for action." Obama has also had to balance conflicting goals. He favors a "public option" - a government-run health program that would compete with private insurers. But he also wants the support of moderate Republicans who oppose the public option. Several congressional panels have been working on health care bills, but only one - the Senate Finance Committee - offered a reasonable prospect for a bipartisan compromise. But the panel's Democratic chairman, Sen. Max Baucus, said Wednesday that the panel would start drafting legislation in two weeks, whether or not Democrats and Republicans have come to an agreement. Liberals have been urging Obama to use his large Democratic majorities in both chambers of Congress to disregard Republican objections and push through a plan with a public option. Some have threatened to vote against any bill that doesn't have one. But it is not clear that Obama can win approval for a government plan, especially since some conservative Democrats are also wary of it. On Wednesday, Obama spoke in favor of the public option. But in a remark certain to displease liberals, he did not insist on it, and said he was open to other alternatives that create choices for consumers.