She firmly refused to provide current enrollment numbers, saying repeatedly they will not be available until mid-November. However, she did try to lower expectations of strong initial sign-up.
Tavenner's appearance followed last week's testimony of outside contractors who said there wasn't enough time to test the complex online enrollment system. It froze up the day it was launched, Oct. 1.
(Read more: Obamacare contractor promises: We'll fix it soon)
At stake is what the Republicans' partial government shutdown could not achieve: a delay of President Barack Obama's law expanding coverage for uninsured Americans. As a result of widespread sign-up problems, even some Democrats have joined Republicans in calling for a one-year postponement of the law's tax penalties for the remaining uninsured. The insurance industry warns that would saddle the new system with too many high-cost patients.
By every indication, initial enrollments have been disappointingly low. Although millions of Americans were interested in checking out new options, apparently few have been able to get through the online application process. "We expect the initial numbers to be small," said Tavenner.
An internal memo obtained by The Associated Press shows that the administration expected nearly 500,000 uninsured people to sign up for coverage just in October, the program's first month. Tavenner repeatedly declined to cite enrollment numbers, saying they will not be provided until mid-November.
House Ways and Means Chairman Dave Camp, R- Mich., drew his own conclusion. He told Tavenner that by his math, the administration appears headed for less than a fourth of its October sign-up estimate.
(Read more: White House knew of Obamacare cancellations)
Less well known than Health and Human Services Secretary Kathleen Sebelius, Tavenner was closer to the day-to-day work of setting up the enrollment website, which was handled by experts within her agency, the Centers for Medicare and Medicaid Services, along with outside contractors. Like other administration officials, she previously had assured Congress that everything was on track for a reasonably smooth launch in all 50 states.
"If people can't navigate such a dysfunctional and overly complex system, is it fair for the IRS to impose tax penalties?" asked Camp. In a concession, the White House has said it will waive penalties for anyone who signs up by March 31, in effect granting a limited grace period.