Millions of uninsured and low-income Americans will sign on to subsidised health coverage that goes on offer today.
It is a revolutionary system that is still working out technical kinks - and is at the centre of a huge political storm.
The Affordable Care Act promises access to health coverage regardless of age, sex or pre-existing conditions.
But opponents of "Obamacare" fear it is a giant leap towards socialised medicine and that it is not ready to be put into action.
Republicans have tried more than 40 times, unsuccessfully, to repeal the law at federal level.
And Obamacare has become linked to a budget stalemate that could lead to the first US government shutdown for 17 years.
Last-minute negotiations were expected to take place in Washington to try to agree a new budget, amid attempts by Republicans to withdraw funding for the health care act.
If they fail to agree by midnight (noon HK time), the government will be forced to close all non-essential federal services.
But the president has said he has no intention of allowing rivals to chip away at his signature domestic legislative achievement. "That's not going to happen as long as I'm president," he said last week.
One thing is certain - while Obamacare becomes a reality today for those eligible to purchase coverage through so-called health insurance exchanges, it remains an imperfect system.
"It's going to be crazy," predicted Alicia Wilson, who runs La Clinica del Pueblo assisting the Hispanic community.
Despite the reform's passage into law more than three years ago, "we're just now being confronted by how much people don't know" about it, she said.
Wilson said her group is demonstrating how elements of the system work so people can begin to understand how to enrol. But she added: "We won't probably complete that many applications in the first month until they finish all the glitches." The daunting, nationwide project is complicated by the government's acknowledgement that the system's websites have had technical problems.
The problems will prevent many from signing up online once the exchanges - essentially marketplaces where consumers can shop around and purchase coverage - open today.
Vincent Keane, president of Unity Health Care, which runs about 30 health centres in Washington, said that despite a bombardment of public announcements, "there's a lot of misinformation out there" from people intent on maligning Obamacare.
In a country where health care coverage is mainly private, more than half of Americans are insured through their employers.
But about 57 million people are not fully insured for medical or hospital care.
They are not poor enough to qualify for welfare, yet may not be wealthy enough to pay for individual policies. And they often end up paying full price for treatment, including for diseases like cancer and diabetes.
Seven million Americans are forecast to apply for coverage through the health care law by next year and 25 million by 2016.
A federal subsidy will be offered, varying according to household income.
And from January 1, nearly all Americans will need to have coverage or pay a fine.
From that date, insurers will no longer be permitted to base their rates strictly on a person's medical history.
Age, location, household size and smoking habits will be the determining cost factors.