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 in the first moments of the first speech, a jab at Republicans that was reciprocated minutes later.
"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 Reid's oft-made statement.
At a cost of nearly $1 trillion, the legislation is designed to extend health care to millions of American 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. As much as 94 percent of the eligible population would wind up covered. 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.
It has taken months to advance the legislation to the floor, Democrats struggling with their own internal divisions as well as Republican opposition.
Democrats control 60 seats in the Senate, precisely the number needed to trump a promised Republican filibuster. While 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 within the measure. 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 Reid rose to speak, the two parties were squabbling 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 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 between zero and 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 between 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.
CBO said for non-group coverage, premiums would rise by between 10 percent and 13 percent on average. But more than half that group is expected to receive federal subsidies that would result in premiums as much as 59 percent less costly on average. Individuals purchasing coverage without any federal assistance would presumably face higher costs, although CBO's letter did not indicate how much more.
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.
But both parties seemed to have political considerations in mind as the day wore on.
The first proposed change to the legislation, offered by Sen. Barbara Mikulski, D-Md., would increase insurance benefits for women, mandating that policies include an annual health screening.
As the health care debate has unfolded in Congress, both parties have spent months vying for the support of women.
Not to be outdone, Republicans issued a statement saying that as written, an advisory committee that recently drew criticism for proposing a delay in routine mammogram screening would have even greater authority.
The two sides also sparred over issues important to seniors, whom polls show are particularly concerned about the impact of health care.
Reid sought Republicans' agreement that Social Security would be protected as debate moved ahead.
Sen. Mike Enzi, R-Wyo., objected, saying Reid had refused to extend the same protection to Medicare.
Not long afterward, Sen. John McCain proposed stripping out a total of $440 billion in Medicare cuts to home health providers, hospitals, hospices and other organizations, saying those reductions could not be sustained politically.
In a slashing attack on the White House and Democrats, the Arizona Republican accused the bill's supporters of resorting to "Bernie Madoff accounting, Enron accounting" to mask the true impact on the deficit.
The House approved its version of the health care bill last month. It would have to be reconciled with any Senate-passed measure before legislation could go to the White House for Obama's signature.