U.S. Health and Human Services Secretary Kathleen Sebelius is resigning after overseeing the botched rollout of President Barack Obama's signature healthcare law, a White House official said on Thursday.

Her departure removes one lightning rod for critics as Obama and nervous Democrats try to retain control of the U.S. Senate in November midterm elections, but Republicans continue to see problems with the Affordable Care Act as a winning issue.

"If the Obama people thought this was going to calm the waters, I think they misread it. I think it's just going to embolden Republicans," said David Yepsen, director of the Paul Simon Public Policy Institute at Southern Illinois University.

The October 1 launch of new Obamacare health insurance marketplaces, which was plagued by computer problems that stymied access for millions of people, has been condemned by Republicans as a step toward socialized medicine.

Obama has chosen Sylvia Mathews Burwell, his budget director, to replace Sebelius, the White House said. Well-known inside Washington, where her appointment was praised by the likes of Republican Senator John McCain, Burwell will have to manage the program through its next major challenges in the height of elections season.

But Burwell is relatively unknown outside the Beltway, and has a "tall order" to fix all the detailed issues with the law, and improve its standing among voters, Yepsen said.

Polls show Obamacare remains unpopular. In March, 46 percent of people said they had an unfavorable view of the law, while 38 percent said they liked it, according to the Kaiser Family Foundation.

Obama was due to announce the change with Sebelius and Burwell at his side at a White House event at 10:45 a.m. EDT on Friday. Sebelius remains on the job until Burwell is confirmed by the Senate, an administration official said.

Sebelius, 65, became the public face for the problem-plagued start to the enrollment period for Obamacare, which was meant to reduce the number of Americans without health insurance and cut into massive U.S. healthcare costs.

When enrollment opened in October, the federal HealthCare.gov website used by consumers in 36 states failed to work for weeks. The White House called in a team of management and technology experts to fix the site, which began working more or less smoothly by December.

Even as she took responsibility for the failures, Obama stuck by Sebelius, brushing aside pressure to fire her.

Burwell, 48, is no stranger to top-level administrative positions, having served as deputy White House chief of staff during the Clinton administration and in top roles at the Treasury Department and the National Economic Council.

She served at the Office of Management and Budget twice, as deputy director under Jack Lew from 1998 to 2001, and took over as director about a year ago. She helped the administration manage its response to a shutdown of the federal government brought on by a budget battle with Republicans in October.

In the intervening years, she worked at the Bill and Melinda Gates Foundation and as head of the Wal-Mart Foundation.

Burwell "seems to have a strong background in management, and that's what we need now," said Timothy Jost, a healthcare expert who teaches at Washington and Lee University.

Sebelius's resignation caps a series of departures by lower-profile officials responsible for implementing the law.

Sebelius testified to Congress about the law as recently as Thursday, giving no sign that she was about to step down.