Key Republican staffers are departing Capitol Hill with the GOP in control of Congress and the White House, raising questions about how the party will advance its ambitious agenda to overhaul the U.S. tax code and health care system.

Senior personnel for the Senate Health, Education, Labor and Pensions Committee have departed and lobbyists say others are preparing to depart the Senate Finance Committee. A handful of health policy staffers for members on those panels have also left for jobs off Capitol Hill and a key individual on the Senate Budget Committee was recently nominated for a senior post at the State Department.

It is not uncommon for staffers of the same party to leave for posts in the administration early on in a new president’s tenure. And many offices have a bench of individuals to promote to the vacant positions. But there are stark differences between when former President Barack Obama took office and the last five months under President Donald Trump.

While much of the work of staff is done behind the scenes, lawmakers are quick to recognize the strong impact they have on advancing policy.

“Your staff are the people who bring you to the dance,” Sen. Pat Roberts of Kansas, a long-serving Republican who has held the position of chairman on several committees, said. “You have to rely on people who have expertise and experience, who have a symbiotic relationship with committee chair and ranking member.”

Look no further than the 2010 health care law to understand the importance of staff in advancing complex and controversial legislation.

Obama swept into office on a promise to revamp the nation’s health care system. During his tenure Congress passed the law Republicans are now struggling to revamp after seven years of promising to repeal it, as Senate Majority Leader Mitch McConnell once said, “root and branch.”

The effort to advance Obama’s health care overhaul was largely successful because it was buoyed by congressional aides with decades of institutional and policy knowledge.

“In 2009, for many House and Senate Democratic staffers it was the culmination of years of work,” Jim Manley, a former aide to the late Sen. Edward M. Kennedy and former Democratic leader Harry Reid, said in a recent interview. “Many had spent much of their careers trying to advance health care reform legislation, which is why many stuck around until the end.”

In the Senate, Liz Fowler, then senior counsel for former Senate Finance Chairman Max Baucus of Montana, became synonymous with the initiative.

Fowler had years of experience in both the private and public sectors. She served as chief counsel for the Senate Finance Committee during the 2003 debate over the Medicare prescription drug benefit and then left for the private sector, only to return to the chamber in 2008.

Outside of the finance panel, Kennedy, who was chairman of the HELP Committee, also had a powerhouse staff. Among them was John McDonough, who helped implement Massachusetts’ health care overhaul in 2006, signed by Republican Gov. Mitt Romney, on which the 2010 law was based.

Republicans also had a roster of health care all-stars, including Rodney Whitlock and Chuck Clapton, who were key in mounting a defense for the party against a bill no GOP members ended up supporting.

But just as Republicans began to prepare to try to repeal and replace Obama’s signature domestic policy achievement, several key staffers who would have been crucial in stewarding the effort through the Senate departed.

Among the biggest losses, several lobbyists say, was the departure of Mary Sumpter Lapinski, who recently left the Senate HELP Committee for a post in the Department of Health and Human Services.

Lapinski was health policy director for the panel and had years of experience on Capitol Hill. She had worked for Sen. Lamar Alexander of Tennessee, the current committee chairman, since 2007 after spending years in private industry.

The HELP Committee has had other departures as well. Kara Townsend, a health staffer on the panel, recently left for a position in the Office of the Assistant Secretary for Planning and Evaluation within HHS.

On the Senate Finance Committee, two key staffers could soon depart. Lobbyists say Kim Brandt, current chief investigative counsel for the panel, and Chris Campbell, staff director for the committee, are both preparing to leave. Since Finance has so much jurisdiction over health care and taxes, those departures would be a major blow to the GOP’s effort to overhaul both the U.S. health care system and the tax code.

A spokeswoman for the Finance panel directed personnel questions to the administration.

To advance both the health care and tax initiatives, Republicans plan to use the budget short-cut maneuver known as reconciliation that would allow legislation to pass the chamber with only a simple majority. That means the support of the Senate Budget Committee will be crucial.

But Eric Ueland, the panel’s current staff director, has been nominated by Trump to be under secretary of State for management. While that nomination could take weeks or even months to advance, his departure would greatly impact the Republican agenda. Ueland’s input would be critical in ensuring legislation complies with the rules governing reconciliation.

A spokesman for the Budget panel declined to comment.

Outside of the committees, top health staffers have departed the offices of Sens. John Thune of South Dakota, Lisa Murkowski of Alaska, Bill Cassidy of Louisiana, Roger Wicker of Mississippi and Johnny Isakson of Georgia.

While former and current aides say staff is crucial to advancing the legislative agenda, they caution that personnel are replaceable and many of the individuals are departing for positions in the administration, meaning they will still have an influence over policy.

There are other aides who can fill in for those departing. On the tax overhaul, for example, the chief tax counsel and deputy staff director for the Senate Finance Committee has decades of experience on the panel. 

Still, in the absence of former aides returning to Capitol Hill, new hires might not have the legislative prowess to navigate the complex and difficult process required to move bills in the Senate.

“The Senate is a place where there is a certain level of difficulty in reaching compromise,” one former staffer said. “Having staff who have been around other fights who know how to reach deals is important.”