WASHINGTON

Trying to earn a second chance, the senior administration official closest to the implementation of the health care law’s malfunctioning enrollment website will answer questions from Congress at the start of a pivotal week.

Medicare Chief Marilyn Tavenner will be questioned today by the House Ways and Means Committee not only on what went wrong with HealthCare.gov, but also whether lawmakers can trust Obama administration promises to have things running efficiently by the end of November.

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.

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?” said Ways and Means Chairman Dave Camp, R-Mich. 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.

Tavenner began her career as a nurse and built a successful record as a hospital executive before entering public service. Seen as a businesslike manager, she has enjoyed support from lawmakers across the political spectrum. Indeed, Republicans are calling for Sebelius to resign, not Tavenner. But the Medicare chief ’s professional reputation is also at stake.

On Monday, a spokeswoman acknowledged Tavenner’s central role. The Medicare agency “has said we are responsible for the issues the website is currently facing,” communications director Julie Bataille said. As administrator, Tavenner “has been in charge of the overall … implementation effort.”

What Tavenner knew about the potential for problems and whom she told will be key questions from lawmakers. Additionally, some are concerned about the security of the HealthCare.gov site. Others worry about unintended consequences from the feverish, hasty work to repair the site.

Sebelius is likely to face some of the same questions Wednesday when she appears before another powerful House panel, the Energy and Commerce Committee.

Momentum to fix the problems has grown since Obama personally acknowledged the problems last week. He sent in management consultant Jeff Zients to assess the situation. By the end of the week, Zients reported that he had two big lists with dozens of needed fixes, and said he was optimistic they could be completed by Nov. 30.