Rick Wilking/Reuters

When Mark Udall lost his Senate seat in the midterm elections, civil libertarians familiar with his efforts to inform Americans about the CIA and NSA had the same thought: Before leaving office, the Colorado Democrat should tell the public about the abuses the government is trying to hide. National-security officials are able to violate the Constitution and various statutes with impunity in large part because they classify their misbehavior as a state secret. It's a neat trick. To expose their lawbreaking, one must first break the law.

But there is a check on this unscrupulous trick.

Members of Congress can reveal classified information in their capacity as legislators without facing legal consequences. As the U.S. Constitution puts it, "The Senators and Representatives shall receive a Compensation for their Services, to be ascertained by Law, and paid out of the Treasury of the United States. They shall in all Cases, except Treason, Felony and Breach of the Peace, be privileged from Arrest during their Attendance at the Session of their respective Houses, and in going to and returning from the same; and for any Speech or Debate in either House, they shall not be questioned in any other Place."

This "Speech or Debate Clause" was most famously invoked in 1971, when Senator Mike Gravel called a late-night subcommittee meeting and entered the Pentagon Papers into the Congressional Record, liberating them for the public and thwarting executive-branch officials who insisted that they should be suppressed.

Today, it's CIA torture and mass surveillance of innocent people that the executive branch wants to hide. It's beyond dispute that Bush administration interrogation tactics were illegal, as is the fact, documented in FISA Court opinions, that the NSA knowingly violated the Fourth Amendment on many occasions. Yet there is a lot about torture and surveillance that Americans still don't know.

Using the Speech or Debate privilege to reveal abuses could be costly for a sitting senator, who'd risk being stripped of his or her clearance to see classified information or even expelled from the Senate for violating its rules. Udall is a lame duck, so his calculus is simpler. He only needs to ask himself what is right. What fulfills his obligations to his constituents, his country, and the oath of office he took to support and defend the Constitution? Preserving his ability to fight for civil liberties another day is no longer an option.

That frame is clarifying: He is now obligated to speak out. I do not reach that conclusion lightly. As a general rule, I believe legislators should be wary of revealing classified information, but the abuses being covered up are clear, radical and corrosive to a democratic society. Consider the details of the torture issue alone:

• The crime is heinous.
• The Obama administration is in clear, flagrant, longstanding breach of its legal obligation to investigate torture and to refer torturers for prosecution.
• The Department of Justice employee tapped to look into the matter didn't even interview many torture victims.
• The Senate Intelligence Committee spent millions of taxpayer dollars and years of its staffers' time producing a 6,300-page report on CIA torture that is being suppressed years after being completed as the CIA itself influences it.
• The current head of the CIA was himself a high-ranking staffer at the agency during the torture years.
• CIA employees spied on the Senate oversight committee as it completed the report.

Until and unless the report is released, the prevailing narrative on torture will remain influenced by the misleading propaganda of torture proponents, increasing the chance that the U.S. will adopt immoral, ineffective, illegal interrogation techniques in a future war or national emergency.