The senior administration officials said they had arrived at the estimate over weeks of contacting insurers and combing through the administration's own enrollment data.
The officials, who would not be identified talking about data that had not been formally released, acknowledged that the number was an estimate and a moving target. Thousands of people are enrolling daily in new health plans on state- and federally-run websites created under the
Still, the figure is a notable data point in one of the most politically damaging fights over the healthcare law this fall. In the weeks after the law's Oct. 1 launch, insurers sent millions of cancellation notices to consumers who had bought insurance on the individual market in previous years. The letters typically told consumers that their plans did not meet new standards set by the law.
Republicans and others seized on the notices as evidence that the law was unworkable and that President Obama had misled the public when he repeatedly promised that consumers would be able to keep their coverage if they liked it.
Obama apologized for having misled people, but administration officials also said that many of the consumers who received the notices would be able to purchase better coverage in the federal marketplace. That argument was difficult to prove in the two months that the federal insurance website, HealthCare.gov, was essentially broken.
The administration also argued that the cancellations affected a relatively small slice of the individual insurance market of roughly 15 million people, although it never confirmed a precise figure. An Associated Press estimate put the number of cancellation notices sent at 3.5 million.
If the 500,000 figure is correct, the administration appears to have avoided one potential pitfall -- the possibility that the number of uninsured people would actually go up as of Jan. 1. The healthcare law already has seen more than 1 million people receive insurance with Medicaid, and another 365,000 had selected insurance plans in the marketplaces as of the end of November, clearing that bar.
December enrollment totals haven't yet been released, but several states, including California, have reported sharply higher enrollments so far this month.
Officials said they were working to reach the 500,000 people, as well as the millions of others in need of insurance, in the new year. Roughly 800 new customer service agents have been added to the 12,000 agents taking calls at 17 sites across the country, officials said. There are 80,000 agents, brokers and navigators walking consumers through the enrollment process.
Insurers have been encouraged to be flexible as consumers rush to sign up for coverage. People who enroll by Monday will receive coverage that begins on Jan. 1. Insurance companies have said they will wait until Jan. 10 for consumers to pay their premium bill.
Open enrollment under the law continues until the end of March, but people who sign up after Dec. 23 will have to wait until the start of the month after they enroll to receive healthcare coverage.