Getting a bill through the Senate remains a big challenge, but even there, the Obama administration has a reasonable chance of corralling the 60 votes it would need to pass legislation more or less on its terms. One wavering Democratic moderate, Senator Ben Nelson of Nebraska, signaled over the weekend that he might be able to go along with one of the compromise proposals under discussion. Senator Olympia J. Snowe, the Maine Republican whose vote would be vital to Mr. Obama, remains deeply engaged in negotiations, and there are indications that one or two other Republicans, like Senator George V. Voinovich of Ohio, might be in play.

Politically, there is an imperative for Democrats to act; they remember well the disastrous political fate that befell them in 1994, when they lost control of the House and Senate after failing to pass a health bill under President Bill Clinton. Rahm Emanuel, the bare-knuckled political operative and former Clinton aide who is now the White House chief of staff, has wasted little time in reminding his fellow Democrats that, as he said in an interview Tuesday, “the inability to act here will have political consequences.”

None of this is to understate the magnitude of the task facing Mr. Obama as he begins a final drive for the legislation with a nationally televised address to Congress on Wednesday night. The size and complexity of the legislation, the deep partisan divide, the undercurrent of concern among voters about whether government is getting too big and intrusive, opposition from special interests  all create land mines that could still blow up the effort.

But even after weeks filled with seemingly ominous portents for Mr. Obama’s ambitions, there is evidence that public opinion remains basically supportive of him. Despite intense controversy over the “public option,” a government-backed insurance plan that would compete with the private sector, a CBS poll at the end of August found that 60 percent of Americans still support the idea, down from 66 percent in July. And half the respondents to the poll said Mr. Obama had better ideas on health care than Republicans, down from 55 percent.

Mr. Obama likes to say that in the 100 years since President Theodore Roosevelt began advocating universal health care, “we’ve never had such broad agreement on what needs to be done.” On Capitol Hill, it is possible to see how a compromise could come together; Mr. Nelson indicated over the weekend that he could back a provision known as a “trigger” to create a public option if private efforts to cover the uninsured failed.