Making the change now means that Burwell's confirmation hearings will be held amid campaigning for the midterm congressional elections in which the law, known as Obamacare, is a prime target for Republicans.

When Obama nominated Burwell last year as his budget director, she went before the Senate as Obama and Republicans were squaring off in another battle: raising the federal debt limit and dealing with the automatic budget cuts known as sequestration.

She was confirmed on a 96-0 vote.

"I'm assuming not much has changed since that time," Obama said today, drawing some laughter from an audience made up mostly of White House staff.

'Bruising' Fight

Jim Manley, a former top aide to Senate Majority Leader Harry Reid, a Nevada Democrat, said even with her past performance with the Senate, any confirmation process for HHS secretary is bound to be "bruising."

"Republicans think that they've seized on a winning narrative when it comes to Obamacare," Manley said in a telephone interview. "They'll be bound and determined to make as much hay out of a confirmation process as they can as they try to put the program in the worst possible light."

While Burwell's previous experience in the confirmation process will help, "I'm not so sure she can expect to get the numbers for confirmation that she got for OMB director," he said. "I assume it will be much more partisan this time."

'Obamacare's Short-Comings'

Senator Mitch McConnell of Kentucky, the Senate Minority Leader, said in an e-mail that Burwell's nomination should be "the start of a candid conversation about Obamacare's short- comings and the need to protect Medicare for today's seniors, their children and their grandchildren."

Another Republican, Senator John McCain of Arizona, said on Twitter that Burwell is "an excellent choice" for the job.

No schedule has been set for confirmation hearings. Sebelius is planning stay in the job into next month, and the White House will press Democrat leaders in the Senate to get hearings set as soon as possible, according to an administration official who asked for anonymity to discuss strategy.

Burwell, 48, is familiar with the territory. She was deputy director of the OMB near the end of the Clinton administration, another time when government control was divided and the partisanship was intense.

Burwell has acknowledged that the difficulties she faces in today's partisan climate are greater than in the 1990s.

"The main difference that I would articulate is the alignment between the Republicans during that period," the West Virginia native said in an interview late last year. That unity "helps one get to solutions," she said.

No Successor

No successor to Burwell was named. Stan Collender, a budget veteran of Capitol Hill, said two of the contenders mentioned most frequently are Jason Furman, chief White House economist, and Jeff Zients, the current director of the National Economic Council who has held OMB posts as deputy director and acting director.

Zients "could step into it pretty quickly," said Collender, now executive vice president of Qorvis MSLGroup, a public affairs firm Washington. Furman, he said, "is economically smart" and an inside player who "has the president's ear."

A graduate of Harvard University and Oxford University as well as a former Rhodes Scholar, Burwell also was President Bill Clinton's deputy chief of staff. Before joining the Clinton White House she was chief of staff to then-Treasury Secretary Robert Rubin.

Foundation Work

After leaving Washington, Burwell worked in philanthropy. She ran the global development program at the Bill and Melinda Gates Foundation. In January, 2012 she was named president of the Wal-Mart Foundation, which gave $959 million in cash and in- kind contributions worldwide in 2011, according to its website. She held the job until Obama chose her for OMB in March, 2013.

Her White House connections are deep. National Security Adviser Susan Rice helped introduce Burwell to her husband, lawyer Stephen Burwell, with whom she has two children. The two women first met as Rhodes Scholars at Oxford University. Housing and Urban Development Secretary Shaun Donovan was a dormitory mate at Harvard.

Budget Matters

At OMB, Burwell oversaw preparation of the government's $3.8 trillion annual budget, representing about one-fourth of the U.S. economy.

One of the reasons Obama picked her to take over HHS was her track record in navigating through the partial government shutdown last October and helping win a two-year budget agreement, according to an administration official, who asked for anonymity because the decision hasn't been announced.

The Health and Human Services Department budget for fiscal year 2015 is just over $1 trillion, more than 80 percent of that for Medicare and Medicaid, the health insurance programs for those over 65 and the poor.

Along with the Centers for Medicare and Medicaid, HHS oversees the Centers for Disease Control and Prevention, the Food and Drug Administration and the National Institutes of Health.

It also plays a central role in administering the Patient Protection and Affordable Care Act. Sebelius told a Senate hearing this week that 7.5 million people had enrolled in private health plans under the law.

–With assistance from Lisa Lerer, Julianna Goldman and Roger Runningen in Washington.

Copyright 2018 Bloomberg. All rights reserved. This material may not be published, broadcast, rewritten, or redistributed.