After President Trump’s personal lawyer, Michael Cohen, pleaded guilty to eight felonies, the president blasted how federal prosecutors coerce people to plead guilty, whether or not they’ve done anything wrong, and to play ball with investigators to target others. Mr. Trump said “it’s called flipping and it almost ought to be illegal.”

A self-serving remark, but he’s got a point.

Legal reformers like Human Rights Watch agree with Mr. Trump on this one, though they’re hardly ever on his side. A staggering 97% of defendants plead guilty because their sentence would be tripled, on average, if they didn’t. Sounds more like North Korea or the Soviet Union than the United States of America.

If Cohen had continued to maintain his innocence, prosecutors were going to pile on the charges so that he faced 65 years behind bars — a life sentence — and the possibility his wife would be charged. In exchange for a guilty plea, he was guaranteed no more than 63 months in prison and possibly none — not a gamble most people would take even if they’d done nothing illegal.

Prosecutors argue that their hardball tactics are far more efficient and cost-effective than taking defendants to trial. That has merit, and it appeals to taxpayers and law-and-order groups. Legal reformers, though, want prosecutors to stop threatening defendants with draconian sentences just for daring to plead innocent.

In America, people are supposed to be considered innocent until the government, in a courtroom, proves its case beyond a reasonable doubt. With plea deals, there’s no standard of evidence, no proof.

The Constitution’s guarantee of a fair trial is disappearing.

“Federal criminal defendants are being coerced to plead guilty,” cautions the nonpartisan National Association of Criminal Defense Lawyers. “There is no more heart wrenching task than explaining” to an innocent person that “they must seriously consider pleading guilty or risk the utter devastation of the remainder of their life.”

Yet flipping has plenty of supporters. Peter Zeidenberg, a former federal prosecutor, claims it’s “fundamental to the process.” Without the threat, prosecutors wouldn’t get defendants to provide information about other possible targets of investigation.

How credible is that information, though, if it’s extorted from a terrified defendant to save his own neck? Prosecutors have total discretion to send a letter to the sentencing judge urging no jail time at all for defendants who tell prosecutors what they want to hear. Defense lawyers caution that this practice “entices defendants to embellish the facts or even lie.”

Cohen’s plea deal made no mention of cooperating with the New York prosecutors who charged him or with special counsel Robert Mueller’s investigation. But that deal’s not written in stone. Cohen’s spokesperson, Lanny Davis, is already telling talk show hosts that Cohen “is more than happy to tell the special counsel all that he knows” — though he’s already walking back some of his promises. If Cohen says enough to implicate the Trump campaign, he could get off with no jail time. How believable is a witness under that kind of pressure?

Former organized crime prosecutor Kenneth McCallion dismisses that concern. Just because Cohen “is trying to get a benefit from his cooperation does not mean that” what he says is “fabricated.” Mr. McCallion knows firsthand how often squeezing defendants to cooperate helps bring down organized crime. He says it’s essential because the legal system “places exceedingly high requirements” on prosecutors to prove guilt beyond a reasonable doubt.

Only in a trial, and those are becoming a rarity. Fewer than 3% of the accused ever get one, whether they’re big shots like Cohen, or an ordinary Joe accused of tax evasion, burglary, assault, or a drug crime.

Mr. Trump’s remark about “flipping” is shining a light on a system that urgently needs reform. Plea bargaining and cooperating defendants are prosecutorial tools, but they’re being abused. The National Association of Criminal Defense Lawyers is urging judges to shrink the penalty defendants face if they insist on a trial. It’s a constitutional right, and it should not come with a horrific price tag.

Ms. McCaughey is a senior fellow at the London Center for Policy Research.