Myriad Agencies In The Federal Government Are Involved
Prior to the creation of the Department of Homeland Security, almost every U.S. government agency had its own law enforcement or security division. Many U.S. government agencies still do.
Prior to 9-11 the Federal Protective Service which provides security in most federal buildings, strictly speaking, for the buildings, but as a practical matter, for all of the federal employees in the buildings, was housed in the General Services Administration, although it has now been relocated to the Department of Homeland Security.
The model of protecting buildings or operations, while in practice, protecting employees of an agency, is also true of many of the other agencies of this type.
Congress has the U.S. Capitol police.
The U.S. Marshals service, while formally housed in the U.S. Department of Justice, primarily works at the direction of the judges and court administrators of the federal courts and protects judges, although they are also the utility infielders of the federal government when it comes to protective duties that aren't in anyone else's jurisdiction. The U.S. Supreme Court also has its own police force.
The Department of Defense (which has more civilian employees than any other federal government agency in addition to having 1.4 million active duty soldiers and more reservists and national guardsmen) has at least twenty-three civilian and military agencies that are detailed to provide security for its bases and offices, in addition to ordinary soldiers on guard duty. A lot of the top brass in the military are protected by the U.S. Army Protective Services Battalion which is part of the U.S. Army Criminal Investigation Command and the United States Pentagon Police, whose counterpart in the Department of Navy is NCIS (the Naval Criminal Investigation Service) made famous by the eponymous police procedural TV series about the agency. There are two more such agencies in the Coast Guard which is in the Department of Homeland Security in peacetime, but is part of the U.S. Navy chain of command when activated for wartime duty.
The CIA has employees with the title of Protective Agents who are basically high end security guards with better security clearances than your average person in that kind of position, and people with the title of Police Officer in the Security Protective Service (it isn't entirely clear to me where Protective Agents fit in the CIA organizational chart).
The Department of State has an elite bodyguard corps, similar in character to the Secret Service but much less well known, called the Diplomatic Security Service (which, in turn, reports to the Undersecretary of State for Management), which protects U.S. diplomats in the foreign service, in addition to Marine guards at U.S. embassies.
The U.S. Postal Service has the U.S. Postal Inspection Service, an investigative law enforcement agency which, while primarily focused on finding plots to harm others through the mail also looks at internal threats., as well as the U.S. Postal Service Office of Inspector General Office of Investigations that focuses on internal threats.
Similarly, the Housing and Urban Development Department has investigative offices in the Office of Inspector General that handles internal threats, and a Protective Service Division.
You wouldn't naturally think so, but a very substantial share of federal employees who aren't in the Department of Defense or Postal Service, are law enforcement officers in the Department of Interior and Department of Agriculture which manages and maintains order upon the vast share of land, mostly in the Western U.S., that is owned by the United States government and managed by agencies such as the Bureau of Land Management, the Forest Service, and the Park Service. Some of these law enforcement officers end up helping to protect their agency's senior officials. The Department of Agriculture also has a Protective Operations Division and an Office of Inspector General that keeps an eye out, among other things, for internal threats like disgruntled employees.
The Commerce Department has an Office of Security that manages the U.S. Commerce Department Police. The Department of Health and Human Services has its own National Institutes of Health Police force.
Individualized threats against federal employees by isolated individuals who have a grudge out of some dealing with an agency are usually referred to the Federal Bureau of Investigation (FBI), because many forms of threats and assaults directed at federal employees are federal crimes.
The White House, as opposed to the President personally, used to have its own police force in addition to the Secret Service to provide security to the President and a few other senior officials. But in 1930, it became part of the United States Secret Service. It was renamed the Executive Protective Service in 1970 and then the Uniformed Division of the Secret Service in 1977. There are also several military details attached to the White House (such as those that operate Air Force One, the Presidential airplane, and Marine Corps One, the Presidential helicopter).
The Threat Is Limited
But you don't have to go very far down the totem pole of the federal bureaucracy at all before civil servants fade into anonymity. Do you know who the assistant deputy secretary of the United States Treasury for administration is? Or what he or she does? Do you know who judge Kavanaugh's law clerks are? Who has something in for the regional manager of the Social Security Administration or the Comptroller of the Currency? Who poses a threat of physical harm to members of the board of directors of the Smithsonian Institution (which, of course, spends much of its payroll on museum security guards and actually has three separate law enforcement agencies of its own)? Who is out to get the Copyright Registrar or the director of the Library of Congress?
Policy wonks and political science professors and Washington Post journalists and lobbyists know, but they aren't people who use physical threats to accomplish their goals in life.
If the public, or focused subsets of the public, don't know who you are, you don't need to worry much about being a target of the kind of random threats that bodyguards are for. They are protected mostly by the fact that what they do, in the eyes of the general public, at the level of detail they deal with, is boring.