Story highlights A GOP lawmaker says Sebelius' departure was "a long time coming"; Pelosi praises her

Sebelius was criticized for Obamacare site's rocky rollout, then cheered its enrollment

Officials: She told the President in early March that she'd resign; it was her decision

Obama plans to nominate OMB director Sylvia Burwell to replace Sebelius, official says

Kathleen Sebelius -- who weathered heavy criticism over the flaw-filled launch of the Obamacare website, then saw the program through as it topped a major milestone -- is resigning as secretary of the Department of Health and Human Services, a White House official said Thursday.

President Barack Obama intends to nominate Sylvia Mathews Burwell, current director of the Office of Management and Budget, to replace Sebelius, according to the official.

A former Kansas governor and, before that, state insurance commissioner, Sebelius was sworn in as HHS secretary in April 2009.

Her time as head of the federal health agency coincided with the passage and implementation of the Affordable Care Act, the bill often referred to as Obamacare.

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Sebelius came under fire last fall for the rocky rollout of HealthCare.gov , the website central to the new law's implementation.

That included being subject of a "Saturday Night Live" parody and talk show one-liners panning her. Republicans in Congress were especially critical of what they saw as her lack of leadership shepherding through what they saw as an ill-conceived, ill-advised law. Wyoming Sen. John Barrasso went so far as to characterize her last October as the "laughingstock of America ."

But Sebelius, 65, held on to her job, insisting America shouldn't abandon the legislation and all that it hopes to achieve.

In an interview with CNN's Sanjay Gupta , she admitted that Obama didn't know of the website's many technical problems until "the first couple of days" after it went live October 1.

"There are people in this country who have waited for decades for affordable health coverage for themselves and their families," Sebelius said, explaining why the website's launch wasn't pushed back despite anticipated problems. "...So waiting is not really an option."

The website's performance did improve significantly, prompting the calls for her job to die down as well. Earlier this month, in a letter to department employees, Sebelius reflected on Obamacare enrollment exceeding its target of 7 million as evidence of "the progress we've made, together," while stating "our work is far from over."

"I know that this law has been at the center of much debate and discourse in Washington, but what this enrollment demonstrates is that the Affordable Care Act is working and much needed," she said in the note.

According to senior Obama administration officials, Sebelius told the President in early March that she thought the enrollment period would end well and, after that, she planned to step down. Even granted the initial uproar over the website, her decision to resign was on her own accord, the officials said.

One White House official praised her overseeing "one of the most consequential initiatives of this administration" as well as her efforts to "improve children's health, expand mental health care, reduce racial and ethnic disparities, bring us closer to the first AIDS-free generation and promote women's health."

"The President is deeply grateful for her service," the official said.

House Majority Leader Eric Cantor, a Virginia Republican, thanked Sebelius for her five years in the federal government -- while taking at a swipe at the legislation she is most closely associated with.

"She had an impossible task: nobody can make Obamacare work," Cantor tweeted

Other Republicans weren't that gracious. Rep. Marsha Blackburn of Tennessee said Sebelius' departure "has been a long time coming after a litany of failures and total mismanagement."

Not surprisingly, given the sharp partisan divide that defines the Obamacare debate, Democrats came to her support. House Minority Leader Nancy Pelosi commended the outgoing health secretary for her dedication "to a single purpose: to make health care a right, not a privilege, for all Americans."

"When all is said and done," tweeted ex-Obama senior adviser David Axelrod , "Sebelius has lots to be proud of, including the surprisingly strong finish on exchange signups after rocky start."

Sebelius is expected to be by the President's side at 11 a.m. Friday when he announces Burwell's nomination, according to a White House official.

Burwell, 48, was confirmed to her current Cabinet-rank position in April 2013. She came to the White House from her spot atop the Walmart Foundation -- the giant retail chain's charitable organization which, according to its website, donated nearly $1 billion to causes worldwide in 2011.

Prior to that, Burwell worked for the Bill and Melinda Gates Foundation and in President Bill Clinton's administration under then-Treasury Secretary Robert Rubin.