The White House said that by the end of the day it hoped to reach its target for seven million applications for “Obamacare” before consumers have to pay a penalty for not doing so.
In an echo of the technical problems which dogged the launch of the service, the website which handled applications went down for six hours yesterday under the weight of traffic.
There were also long queues from 5am at health service centres across the United States for those signing up in person.
Despite this, Republican Senator John Barrosso claimed the government was “cooking the books” and renewed his party’s vow to repeal the Patient Protection and Affordable Care Act, the policy’s official title, which they see as the ultimate example of the nanny state.
If the Republican Party seizes control of the Senate in the mid-term elections in November, it could force Mr Obama to make key concessions, threatening a policy he sees as his legacy.
The deadline comes after a six-month enrolment window, a milestone in the evolution of a law the president spent much of his political capital to pass back in 2010.
However, Mr Obama said those who had indicated they would apply but did not complete their application by last night would get another week to do so. About 1.2 million people visited the official website healthcare.gov on Saturday and on Sunday queues formed at 5am at a health centre in Miami.
US political website Politico reported that Mr Obama, vice-president Joe Biden and other top officials have done more than 300 radio interviews in the last weeks to drum up support.
By meeting the target of seven million applicants by 31 March, it showed “the law’s working”, said White House senior adviser David Plouffe. He added: “This was a seminal achievement.”
The Affordable Care Act is designed to ensure many of the 48 million Americans do not have health insurance are covered. At present, Americans are not obliged to buy health insurance, though many are covered by premiums taken out on their behalf by their employers. The poorest are covered by Medicaid, a basic healthcare package provided by the government.
Obamacare will for the first time compel everyone who is not covered by their employer’s insurance and can afford to pay to buy a premium through a health exchange, though many policies will be subsidised.
If they do not, they face a fine of $95 (£57) or 1 per cent of their salary, whichever is greater.
The law also aims to slow the growth in the cost of healthcare through various measures and requires private plans to meet a certain level of coverage.
The deadline day could prove one of the brightest moments for the legislation if Republicans take back the Senate, which looks possible. During a special election in Florida last month – the equivalent of a UK by-election – Republican candidate David Jolly won in a campaign that was more about national healthcare than local issues.
Republican strategists said that the result was “a loud warning for other Democrats running coast to coast”.
The government came close to shutting down last year when hardline Republicans refused to let Obamacare pass unless the US government debt was reduced, though they later relented.
Republicans will almost certainly not be able to repeal Obamacare if they win control of the Senate, but they could change some of its key – and least popular – requirements, including the demand that most individuals obtain coverage and employers with 50 or more workers offer health coverage.
Among the other controversies are the cost of the premiums, with some reports claiming consumers will be paying double what they previously paid.
The White House has not been helped by the bungled launch of healthcare.gov – due to glitches, just six people were signed up on the first day, a number predicted by satirical comedy TV show Saturday Night Live.