A peek into state data reveals vast disparities. Florida, the state with the third-highest number of uninsured (behind California and Texas), had the most enrollees in the federal-run exchange, 3,571. Texas was second, with 2,991. There were just 42 in North Dakota.

In explaining the relatively low figures, administration officials cite problems with the federal website that have prevented people from signing up. But they also say experience shows people wait until the last minute.

When Massachusetts expanded health coverage in 2007, only 123 of the 36,167 people who ultimately signed up did so during the first month of enrollment. But more than 7,000 signed up in the final month. (Massachusetts counted only people who had already paid their premiums as enrollees, according to Jon Kingsdale, who ran that state’s health insurance exchange for the first four years.)

There is still one month to go until the Dec. 15 deadline for signing up for coverage that begins Jan. 1; the initial enrollment period does not close until March 31. So administration officials, and some outside experts, say these early figures do not reveal much.

“These numbers are interesting,” David Simas, a top White House adviser on health care, said in an interview, “but in terms of any kind of insight into the success of the program, they’re not the central indicator.”

In political terms, though, the numbers are yet another problem for the White House, which is one reason Ms. Sebelius — and not the president — announced them. Republicans insist that, at this rate, there is no way the administration can reach its goal.

“By the time we reach the critical month of December, actual enrollment could lag projections by over one million people,” Representative Dave Camp, Republican of Michigan and chairman of the House Ways and Means Committee, wrote in a letter this month, accompanying a subpoena for detailed enrollment data.