A key component of Barack Obama's landmark health care overhaul has begun despite the federal government shutdown sparked by Republicans trying to derail the law designed to extend insurance coverage to millions of Americans.
People will be able to shop for the first time on insurance marketplaces that open around the US, getting their first close look at "Obamacare" - the programme at the centre of the political storm in Washington.
The online insurance exchanges open after months of buildup as President Obama and his allies tried to convince sceptical Americans of the merits of the plan that could define his legacy.
Republicans have seized on a list of technical glitches and delays that have emerged as evidence the law is not ready to work and will never be. But the White House said that was true for any big, new programme and will not affect the outcome, since Americans have six months to enrol through the exchanges.
"I would suspect that there will be glitches. This is 50 states, a lot of people signing up for something. And there are going to be problems," Mr Obama said. "But what we're confident about is that people will be able to take a look and find out whether this is something that is going to be good for their families."
The government shutdown will have no immediate effect on the insurance marketplaces that are the backbone of the law, because they operate with money that is not subject to the annual budget wrangling in Washington.
The marketplaces opening in all 50 states represent a turning point in the US approach to health care, the biggest expansion in coverage in nearly 50 years and the closest the country has come to universal coverage after a century of efforts.
Republicans vehemently oppose the law, especially a mandate that all Americans have health care insurance or face tax penalties. The law provides subsidies to help lower-income people pay for the plans.
With the exchanges opening, consumers will get a chance to evaluate the premiums and the out-of-pocket costs of the plans offered to them.
The Obama administration hopes to sign up seven million people during the first year and has a goal of eventually signing up at least half of the nearly 50 million uninsured Americans through an expansion of Medicaid, the government-funded programme that provides health care coverage for poorer Americans, or government-subsidized plans provided by private insurers.
But if people become frustrated with predicted glitches in the computer-based enrollment process and turn away from the program, the prospects for success could dim.
One of the biggest challenges to the law's success is the ability of insurers to persuade relatively young and healthy people to buy insurance, as a way to balance the costs for the sicker people who are likely to get coverage as quickly as possible.
Under the law, health insurance companies can no longer deny coverage to someone with a pre-existing medical condition and cannot impose lifetime caps on coverage. They also must cover a list of essential services, ranging from mental health treatment to maternity services.