The day may come when the worst nightmare a crooked banker or compromised regulator can have begins with the words, "You have a letter from Senator Warren."

But before we get to that, here's an experience that may seem familiar: You're at a party or family get-together - a Sunday barbecue, perhaps - and someone says something like, "We need less government regulation." Next thing you know you're having an argument.Here's some advice for the next social event: There's no need to get into an argument. You can just ask, "How do you figure?"

With every unreasonable assertion you can ask a reality-based question like, "Where's the study that says that?" Once in a while they may cite a shallow white paper from sine right-wing foundation, but more often than that they won't even get that far. Soon the conversation will peter out with a "Well, uh ..."We can never go wrong asking questions. We only go wrong when we don't ask questions.

That's what makes this letter from Sen. Elizabeth Warren so important. For five years we've watched the Justice Department ignore overwhelming evidence of bank crime, on grounds that Attorney General Eric Holder made explicit only last March when he said that "the size of some of these institutions becomes so large that it does become difficult for us to prosecute them when we are hit with indications that if you do prosecute... it will have a negative impact on the national economy, perhaps even the world economy."

The Securities and Exchange Commission, which has responsibility for pursuing civil bank fraud, has taken the same approach. So has the Federal Reserve, which has regulatory responsibility for the banking industry. They've all been saying pretty much the same thing: That criminal prosecution would destabilize the financial sector and put the world's economy at risk.

With this letter, Sen. Warren is asking these agencies a very simple question: "How do you figure?"

Warren has already asked it of Thomas J. Curry, who runs the Office of the Comptroller of the Currency and who has taken the same hands-off approach. She didn't put it that way, of course. What she asked him, for the record, was:

"Has the OCC conducted any internal research or analysis on the trade-offs to the public between settling an enforcement action without admission of guilt and going forward with litigation as necessary to obtain such admission? If so, can you provide that analysis to the Committee?"

She got pretty much the same answer from the OCC as you might get at that barbecue: "Well, uh ..." What the OCC actually said was "The OCC does not have any internal research or analysis on the trade-offs of settling without an admission of liability."

Really? So we've been watching as billions of dollars worth of fines and settlements exchange hands - with no admission of wrongdoing - for fraudulent behavior that in many cases then continues unabated, and you haven't even checked to see if it's a good idea or not?The mind boggles.

Warren's letter to Holder, Fed chair Ben Bernanke, and SEC Chair Mary Jo White, is remarkably evenhanded. "There is no question," writes Warren," that settlements, fines, consent orders, and cease and desist order are important enforcement tools, and that trials are expensive ... and are often less preferable ..."But," continues Warren, "I believe strongly that if a regulator reveals itself to be unwilling to take large financial institutions all the way to trial ... the regulator has less leverage ... The consequence, can be insufficient compensation to those who are harmed by illegal activity and inadequate deterrence of future violations."

"Strong belief" is, if anything, too mild a term for something which has now been demonstrated over and over and over: Banks conclude these deals on very favorable terms, then continue to commit the same misdeeds.

Meanwhile, the government institutions which regulate banking are in the middle of an epidemic of (mostly) legalized corruption. The revolving door between regulatory agencies and Wall Street is stained with the moral bankruptcy of those who pass through it every day. That includes outgoing Deputy Attorney General Lanny Breuer, who was derelict in his duty to prosecute big banks ... and who now enjoys a richly-paid perch at his former firm, Wall Street defense attorneys Covington & Burling, which was also Attorney General Holder's firm before he entered "public service."

Outgoing SEC Chair Mary Schapiro objected to the "revolving door" term and offered this by way of explanation: "In my case, there's no revolving door... I won't be going back to government." We stand corrected. For people like Schapiro (and presumably Lanny Breuer) who won't be darkening government's doorstep again, the public trust isn't a "revolving door."

It's a doormat.

Sen. Warren has already received an answer of sorts - more of an evasion, really - from Attorney General Holder. The Senator's letter was sent on May 14. On May 16 Holder tried to retract his "too big to jail remarks" with a very unconvincing walkback in which he said he had been "misconstrued" and that "there is no bank, there's no institution, there's no individual who cannot be investigated and prosecuted by the United States Department of Justice."

Banks can be prosecuted, says Holder. It just so happens that none of them have been. So he doesn't need to answer the question anymore, right?

Wrong. The Attorney General must be held to account for his Department's failure to pursue wrongdoing among his past (and, probably, future) clients. The Fed and the SEC must equally be held to task.

The Senator's letter should be the start of a public conversation. But that will only happen if Sen. Warren gets widespread and very vocal support. Without it, this important line of inquiry will disappear under a mountain of trivia. That would be tragic.

This should be the start of a long-overdue public interrogation of our failed regulatory agencies. With any luck, the phrase "a letter from Sen. Warren" will soon carry the same weight that the phrase "60 Minutes is at the door" had in that program's past days of honor.

Let's hope so, because a lot of follow-up questions need to be asked, including: Why haven't any individuals at major institutions been prosecuted for these crimes, since that wouldn't put the institution itself at risk.

Sen. Warren's letter could help build momentum to fight Wall Street's and Washington's legal - and illegal - criminality. But no one person, including a United States Senator, can do it alone. It's going to take a movement - in the halls of Congress, in the boardrooms, and in the streets. But it can be done. Whenever somebody says otherwise, don't argue. Just ask them a simple question:

How do you figure?