Debate is expected to last for weeks over the legislation, which includes a first-time requirement for most Americans to carry insurance and a mandate for insurers to cover any paying customer regardless of medical history or condition.
"We must avoid the temptation to drown in distractions and distortions," Senate Majority Leader Harry Reid said.
"Well, I don't know what's more preposterous: saying that this plan 'saves Medicare' or thinking that people will actually believe you," Sen. Mitch McConnell of Kentucky, said of Mr. Reid's oft-made statement.
At a cost of nearly $1 trillion, the legislation is designed to extend health care to millions who lack it, abolish insurance industry practices such as denying coverage based on pre-existing conditions and cut back on the rise of health care spending overall.
Despite its huge price tag, the Congressional Budget Office has estimated the 2,074-page bill would reduce federal deficits by $130 billion over the next decade. In all, CBO said 31 million uninsured individuals would receive insurance if the bill were enacted, many of them assisted by federal subsidies.
The legislation would be paid for through a combination of cuts in projected Medicare payments to hospitals and other providers, a payroll tax on the wealthy and taxes on drug makers, medical device manufacturers, owners of high-cost insurance and others.
While Mr. Reid spent most of the day jousting with Republicans, his ability to steer the bill to passage will depend on finding ways to finesse controversial provisions.
None is more important than calls for the government to sell insurance in competition with private firms. Liberals favor the plan; moderate and conservative Democrats oppose it. As drafted the bill establishes a so-called government option, although each state can block it.
Even before Mr. Reid spoke, the two parties squabbled over a new Congressional Budget Office study assessing the legislation's impact on the cost of insurance.
Sen. Max Baucus, D-Mont., said it showed that "whether you work for a small business, a large company or you work for yourself, the vast majority of Americans will see lower premiums than they would if we don't pass health reform."
Not so, said a statement from Mr. McConnell's office: "Most people will end up paying more or seeing no significant savings."
The 28-page report was less clear-cut than either side said. It said that by 2016, premium prices for Americans working at large companies, about 134 million people, would be zero to 3 percent lower on average than would otherwise be the case.
At small companies, estimated to provide coverage for 25 million by 2016, the average premium would be 1 percent higher to 2 percent lower on average. That did not factor in the federal subsidies that would be available to the firms to spur them to provide coverage. Those receiving the assistance would have premiums as much as 11 percent lower on average.
The debate over premiums was only one of many expected as the Senate dug into a complicated bill that seemingly delved into every corner of the health care system.