WASHINGTON — The vote on Monday, in the dead of night, was 60 to 40. The vote on Tuesday, just after daybreak, was 60 to 39. And the vote on Wednesday afternoon, at a civil hour but after less-than-civil debate, was 60 to 39 again — an immutable tally that showed Democrats unwavering in the march to adopt a far-reaching overhaul of the health care system over united Republican opposition.
The votes also marked something else: the culmination of more than a generation of partisan polarization of the American political system, and a precipitous decline in collegiality and collaboration in governing that seemed to move in inverse proportion to a rising influence of lobbying, money, the 24-hour news cycle and hostilities on talk shows and in the blogosphere.
The health care legislation is likely to be approved Thursday morning, with the Senate divided on party lines — something that has not happened in modern times on so important a shift in domestic policy, or on major legislation of any kind, lawmakers and Congressional historians said.
The Democrats flaunted their unity on Wednesday at a news conference with nearly their entire caucus in attendance.
Many senators said the current vitriol, which continued on the floor on Wednesday with a fight over when to cast the final health care vote, was unlike anything they had seen. "It has gotten so much more partisan," said Senator John D. Rockefeller IV, Democrat of West Virginia. "This was so wicked. This was so venal."
Even in a bitter fight over President Bill Clinton's budget in 1993, decided 51 to 50 with a rare tie-breaker vote by Vice President Al Gore, the partisanship was not as stark as it is today. Although no Republicans voted for Mr. Clinton's budget, six Democrats joined them in what amounted to bipartisan opposition.
Mr. Rockefeller said the health bill had created an almost perfect storm of political and policy disagreements, so that some of the bitterness reflected basic philosophical disputes crystallized by President Obama's agenda. "If there was ever a time for that kind of partisanship to come out, this was the bill to do it," he said.
Ross K. Baker, a political scientist at Rutgers University and an expert on the history of the Senate, said that in earlier eras, senators would routinely cross party lines to vote in favor of major legislation on issues like civil rights and social welfare policy.
In 1965, the Senate created the Medicare program by a vote of 68 to 21, with 13 Republicans joining 55 Democrats in favor, and 7 Democrats joining 14 Republicans in opposition. In 2003, some Democrats in both the House and the Senate voted with most Republicans to add a prescription drug benefit to Medicare.
"It certainly is a culmination of a long period of intensifying political polarization," Mr. Baker said of this year's showdown over health care. "It has gotten so bad now that Republicans don't want to be seen publicly in the presence of Democrats or have a Democrat profess friendship for them or vice versa."
With Democrats nominally controlling 60 seats, the precise number needed to overcome Republican filibusters, there is no room for wavering Democrats to break ranks. If they held one less seat, there would be no choice but to win over a Republican; one or two more, and one or two senators with apprehensions could be released to vote no.
Some lawmakers predicted that the Senate would eventually rediscover its genteel equilibrium.
"There's a tolerance level here for what we have just been through, and I think we have hit the tipping point," said Senator Christopher J. Dodd, Democrat of Connecticut. "It got rougher than it should. We are getting precariously close to fracturing an institution where no one wins, so I think we are going to be back on track."
But some experts said that the divide in the Senate reflected a broader political shift that lawmakers cannot easily reverse. "In the 1970s, for instance, there was a much wider political spectrum in both parties," said Donald A. Ritchie, the Senate historian. "You had conservative and liberal wings in both parties."
Mr. Ritchie and many senators said they had witnessed the change in the last 30 years.
"You have got this divide, this polarization in America," said Senator Olympia J. Snowe of Maine, the only Republican in recent weeks to seriously consider supporting the health bill. "People become risk-averse, politically risk-averse. There is no incentive to reach across the divide and appeal to a broader inclination. It looks like pragmatism is a political cop-out; compromise is certainly viewed that way."
But even as senators complained about the rancor and expressed nostalgia for a kinder era, they conceded that the hyper-partisanship was likely to continue, potentially coloring coming debates on other major issues including financial regulation, climate change and, perhaps, immigration.
Senator Max Baucus, Democrat of Montana, and chairman of the Finance Committee, said the political — and often personal — divisions that now characterize the Senate were epitomized by the empty tables in the senators' private dining room, a place where members of both parties used to break bread.
"Nobody goes there anymore," Mr. Baucus said. "When I was here 10, 15, 30 years ago, that was the place you would go to talk to senators, let your hair down, just kind of compare notes, no spouses allowed, no staff, nobody. It is now empty."
For more than 30 years, the major parties — Democrats and Republicans — worked every angle to transform politics into a zero-sum numbers game. State legislatures redrew Congressional districts to take advantage of party affiliation in the local population. The two-year campaign cycle became a never-ending one.
Senator Orrin G. Hatch, Republican of Utah, who worked on many bipartisan health care bills over the years, often with a close friend, the late Senator Edward M. Kennedy, Democrat of Massachusetts, said that the both parties were to blame but that external factors including ethics rules also discouraged senators from fraternizing.
"Both parties have become very polarized," Mr. Hatch said. "A lot of that is because of the stupid ethics rules. We can't get together at various events. A lot of people complain about taking foreign trips, which are really critical for us to understand foreign policy. The Internet is constantly badgering everybody. In the process, it's gotten pretty doggone partisan, both ways. It's bad."
Mr. Hatch and Republican leaders said the lack of any support on their side showed that the health bill was mortally flawed.
The majority leader, Harry Reid of Nevada, at a news conference on Wednesday with most of his caucus standing behind him, offered a different take.
"I don't see this as 60 Democrats versus 40 Republicans," he said. "I see it as 60 leaders who stood up to insurance companies and stood up for working families all across America."
