“Dear Ms. Coe,” an archivist at Marquette University, Senator Joseph McCarthy’s alma mater, wrote in an email, “I regret to inform you that all post-1946 correspondence is sealed for the lifetime of McCarthy’s daughter.” The archivist pasted a link to the collection’s finding aid before concluding, “She has not responded to any inquiries on this matter.”

I was trying to gain access to the senator’s papers for a book project. I’ve worked in lots of government archives, and while the bureaucracy can be a hassle, I usually find what I need. Not this time: Short of suing Senator McCarthy’s daughter or Marquette, I have no real means of recourse.

That’s because the Freedom of Information Act does not apply to the legislative branch. When Congress enacted the F.O.I.A. in 1966, it was after the executive branch’s records, and tailored the law to fit its needs. (Not that the executive branch has done a great job: According to the Center for Effective Government’s 2014 F.O.I.A. Access to Information Scorecard, only eight out of 15 government agencies earned “passing grades.”)

Why are there different rules for the executive and legislative branches of government? The obvious answer is a disheartening one: Congress makes the rules, and its members have chosen to exempt themselves.

The senators and representatives we elect and compensate for their service have marked their professional papers “personal.” If we’re lucky, their archives end up in the right hands — libraries or historical associations — but even then, there are complications. Collections can be sealed for an indefinite amount of time (while a public or private institution foots the bill) or kept in the family, where they may not be properly preserved; either way, access is left to chance. ...