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The Center for Medicare and Medicaid Services is the federal government’s largest agency. It runs on a budget of nearly 1 trillion taxpayer dollars and oversees the two programs largely considered to pose the greatest long-term threat to the federal budget.

President Obama has re-nominated acting Medicare administrator Marilyn Tavenner to head the agency.

But for nearly a decade now, this hulking agency has been without a confirmed director.

Some nominees failed to survive confirmation; others didn’t even get a hearing as senators assumed the process would prove futile. The nomination process has become so politically fraught that Tom Scully, who ran Medicare under President George W. Bush, argues that "Mother Theresa or Gandhi couldn't even get confirmed" as of late.

Last week, Scully decided it was time to break a seven-year streak with no confirmed Medicare head. He joined up with six other former Medicare heads - three Republicans, three Democrats - to plead with top senators to confirm the White House's newest nominee, Marilyn Tavenner.

“Every major discussion of fiscal policy centers on these programs,” the group wrote in a letter delivered to Senate Finance Committee leadership on Wednesday afternoon. “It is important to the performance of Medicare, Medicaid and CMS’s many other functions to have a confirmed Administrator who can provide steady and consistent leadership.”

This nomination is Tavenner’s second. The White House recommended her for the post in November 2011 but, with the fight over the Affordable Care Act still winding its way up to the Supreme Court, she never received a confirmation hearing.

This nomination is already shaping up to be different: A spokeswoman for Senate Finance Chair Sen. Max Baucus (D-Mont.) says the committee will move forward on her nomination after it receives the appropriate paperwork.

“Sen. Baucus intends to hold a hearing,” spokeswoman Meaghan Smith said.

As to why the Medicare administrator post has proved such a tough position to fill, many point a finger at the tough politics of the programs it oversees. It has handled the implementation of a number of controversial laws, including President George W. Bush’s prescription drug coverage program, Medicare Part D, and, most recently, the Affordable Care Act.

"For the last several years we’ve had trench warfare between the parties over health care issues," Bill Roper, who ran the agency under President Ronald Reagan, says. "Advise and consent with the Senate has become a way to wage that warfare."

Both sides have engaged in the fight: Kerry Weems's nomination by President George W. Bush was scuttled by a fight over the Children's Health Insurance Program, which the president vetoed in October 2007.

"Given the vituperative, ideologic nature of that debate, I think Democratic senators felt like they couldn’t give their blessing to anything in the CMS world sought by the Bush administration," Weems recounted in an interview published in the journal Health Affairs. "The Senate Finance Committee had made it clear that my confirmation was unlikely."

Don Berwick, the Obama administration's first pick for the post, faced even more intense opposition. The former Harvard professor came under fire from Republicans for his remarks about Britain’s National Health Care Service, which he described as an “example” that the United States could follow. The Obama administration recess appointed Berwick in the summer of 2010.

In March 2011, 42 Senate Republicans committed to blocking Berwick should his confirmation move forward and, in December of that year, the White House withdrew his nomination.

Donald Berwick, right, Administrator of the Centers for Medicare and Medicaid Services, speaks during a roundtable discussion about health care at Denver Health in Denver on June 22, 2011. U.S. Secretary of Health and Human Services Kathleen Sebelius looks on at left. (Ed Andrieski -- AP)

Even if the administration had settled on a different pick, former administrator Gail Wilenksy thinks it would have been impossible to move any nominee through confirmation process right after a year-long legislative battle over the Affordable Care Act.

“It would have been very difficult for anyone nominated because there was so much anger and frustration,” Wilensky, who ran Medicare from 1990 to 1992 under President George H.W. Bush, said.

Tavenner has so far proved a less controversial nominee than Berwick. She has served as acting Medicare administrator since her nomination in December 2011, overseeing many key regulations for the Affordable Care Act and Medicare, which will ultimately shape the structure of the country's major entitlement programs.

Becoming a confirmed administrator, former Medicare heads say, would give Tavenner more clout, both within the agency as well as in negotiations with other agency heads.

"The basic work will get done either way," Bruce Vladeck, President Bill Clinton's former Medicare administrator, says. "What's really important is it gives someone whose primary concern is Medicare and Medicaid a little more ability and standing to push back against others in the executive branch. The stature of a senate confirmation does give a little extra standing."

Vladeck recalls using that clout when the White House asked him to help assuage some governors who were politically important to the administration, noting that "it comes in handy every now and then."

Confirmation could also give Tavenner more leeway in drafting more aggressive regulations, ones that might be eschewed by an acting administrator whose tenure is temporary.

"When you have an acting administrator, you tend to weaken the ability of the agency to make bold decisions in terms of regulations," Wilensky said. "It pushes them to act in a more conservative way."

This is especially true, Wilensky says, for an administrator like Tavenner who, in awaiting a hearing, may "try to stay out of the public eye and not to make controversial statements."

Tavenner spent most of her career as a hospital administrator before joining the Obama administration in April 2010. She also served as Virginia's Medicaid director under then-Gov. Tim Kaine. She has received endorsements from many health-care organizations, including the American Hospital Association and the American Medical Association.

House Majority leader Eric Cantor also offered an endorsement of Tavenner after her first nomination. The two have known each other since the mid-1990s, when he represented part of Richmond in the Virginia House of Delegates and she ran a major hospital in the city. While the House will not have a say in Tavenner's nomination, Cantor's endorsement could encourage senators to move forward.

As a Republican, Scully fully expects Tavenner will make policy decisions he disagrees with. Even so, he thinks it's time for the Senate to confirm a Medicare administrator - and, under the Obama administration, Tavenner is the best they'll get.

"There's nobody in the universe who should be more confirmable than Marilyn," he says. "I don't know anybody who doesn't like her. If she can't get confirmed, I don't think anyone ever will."

KLIFF NOTES: Top health policy reads from around the Web.

A $2 billion Medicaid program mostly helps undocumented workers. "a little-known part of the state-federal health insurance program for the poor has long paid about $2 billion a year for emergency treatment for a group of patients who, according to hospitals, mostly comprise illegal immigrants. Most of it goes to reimburse hospitals for delivering babies for women who show up in their emergency rooms, according to interviews with hospital officials and studies." Phil Galewitz in Kaiser Health News.

PhRMA is no fan of President Obama's proposal to cut drug spending. "'The president’s proposal to tamper with a program that works well would not yield any benefit for seniors,' PhRMA said. 'Instead, analysts have projected that the president’s scheme would harm Part D’s competitive dynamics, yielding higher premiums, more restrictive access to medicines and diminished research on the next generation of medicines.'" Sam Baker in the Hill.

How to market a health insurance exchange. "In the District of Columbia, everyone from church clergy to city accountants to doctors, got together to sketch a strategy for doing that. The emphasis was on collaboration: if people can get information about the exchange from almost any source, whether it’s their doctor’s office or their church or the health department, they’re more likely to get the message. With as many as 63,000 District residents going without insurance at some point during the year, you need to cast as wide of a net as possible." Dylan Scott in Governing.