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President Obama tapped a new lieutenant on Friday to lead the vast array of federal health programs, saying goodbye to Health and Human Services Secretary Kathleen Sebelius who is resigning after a long and rocky tenure with the administration.

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Sebelius announced her resignation just days after the ObamaCare enrollment period drew to a close. She and the rest of the administration have touted the more than 7 million people who have signed up under the Affordable Care Act exchanges, but the system's disastrous launch last fall and other lingering concerns with the ObamaCare rollout had become a heavy weight on the outgoing secretary.

To replace her, Obama nominated White House budget office director Sylvia Matthews Burwell.

Obama, speaking in the Rose Garden, thanked Sebelius Friday for her "extraordinary service" and said that despite the rocky rollout, Sebelius' team "turned the corner, got it fixed, got the job done, and the final score speaks for itself."

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    In her farewell remarks, Sebelius said they were "on the front lines of a long overdue national change."

    Burwell is subject to Senate confirmation. It's unclear whether that will be a contentious process. She originally was confirmed to the White House post on a 96-0 vote a year ago. Sen. Jeff Sessions, R-Ala., raised early concerns on Friday that Burwell "may have been chosen because the president believed her to be another political loyalist" and that she "has a comparatively thin resume for the demands now placed on this position."

    But aside from the nomination debate, the controversy over ObamaCare, as the midterm election season moves into full swing, is far from over. Republicans continue to warn about premium spikes and other problems on the horizon.

    "Regardless of the administration's public explanation for the Secretary's exit, Obamacare has been a rolling disaster and her resignation is cold comfort to the millions of Americans who were deceived about what it would mean for them and their families," Senate Republican Leader Mitch McConnell said in a statement. "Countless Americans have unexpectedly been forced out of the plans they had and liked, are now shouldering dramatically higher premiums, and can no longer use the doctors and hospitals they choose. .... Secretary Sebelius may be leaving, but the problems with this law and the impact it's having on our constituents aren't."

    The Harvard-educated Burwell was confirmed as White House budget director last April, and previously worked at the top levels of major foundations, including as president of the Walmart Foundation.

    West Virginia Democratic Sen. Joe Manchin praised the nomination of Burwell, a fellow West Virginia native, in a statement Thursday.

    "I am confident that her leadership will ensure that we enact commonsense fixes to the Affordable Care Act to help improve the lives of millions of Americans," Manchin said.

    As Republicans pounced on Sebelius' departure, Democrats defended her.

    House Minority Leader Nancy Pelosi praised Sebelius' leadership during the rollout, saying she had "been forceful, effective, and essential."

    "Her legacy will be found in the 7.5 million Americans signed up on the marketplaces so far, the 3.1 million people covered on their parents' plans, and the millions more gaining coverage through the expansion of Medicaid," Pelosi, D-Calif., said.

    A White House official said Sebelius notified Obama of her decision to leave in early March.

    "At that time, Secretary Sebelius told the president that she felt confident in the trajectory for enrollment and implementation of the Affordable Care Act, and that she believed that once open enrollment ended it would be the right time to transition the department to new leadership," the official said, adding the president "is deeply grateful for her service."

    Sebelius, having served five years with the president, was among the longest-serving Cabinet secretaries in the administration.

    But Sebelius' relationship with the White House frayed during last fall's rollout of the insurance exchanges that are at the center of the sweeping overhaul. The president and his top advisers said they were frustrated by what they considered to be a lack of information from HHS over the extent of the website troubles.

    The White House sent management expert Jeffrey Zients to oversee a rescue operation that turned things around by the end of November.

    The Associated Press contributed to this report.