Former New York state Assembly speaker Sheldon Silver slumps in the back seat of a car outside a federal courthouse in January 2015, while facing bribery charges. (AP photo)

It’s horrifying to watch federal courts institutionalize corruption, redefining it so narrowly that many of the schemes crooked politicians cook up to enrich themselves are now considered legal — or even forgiven after the fact.

It started, arguably, in 2010, when the U.S. Supreme Court freed Enron CEO Jeffrey Skilling from prison 10 years early and questioned whether he was even guilty. The court said the honest services fraud statute only covers blatant bribes and kickbacks, and that more subtle tit-for-tat, pay-to-play favors were essentially OK.

That Skilling decision was then used as precedent that same year to overturn convictions of Conrad Black, a newspaper mogul found guilty of defrauding his media company, and Alaska state Rep. Bruce Weyhrauch, who voted on a tax law bill without disclosing that he was seeking a job at a company with a stake in that law.

Those decisions ended the era when federal prosecutors used the honest services fraud statute to uphold the higher level of ethics that Americans generally expect from their leaders. It’s important to bear in mind that 2010 was also the year of the Supreme Court’s Citizens United v. Federal Election Commission ruling that wiped out campaign finance reform rules and ushered in the modern era of humongous-money politics.

Then last summer, the Supreme Court overturned the conviction of former Virginia governor Bob McDonnell. The Republican was convicted in 2014 of using his job, with its powerful influence, to arrange meetings and do other favors for a diet-supplement mogul who had given him and his wife gifts and loans.

On Thursday, however, things got even worse as a federal appeals court threw out a jury’s conviction of former New York Assembly speaker Sheldon Silver, the ultimate insider in a state capitol infamous for sleaze. Prosecutors clearly showed that the Democrat known as “the Sphinx” rigged up elaborate, ongoing kickback schemes through a cancer researcher and real estate developers — cleverly engineered bribery machines, essentially — that enriched him by $4 million before he was caught.

The appeals judges admitted there was plenty of damning evidence against Silver, but they let him off the hook anyway because they said the court should have included the McDonnell precedent in the jury’s instructions.

It was disgusting. It chopped down New Yorkers’ faith in government even farther than its already low level. Of all the crooked politicians who have ever skulked through capitol corridors, Shelly Silver especially was caught red-handed, and his scheme was more greedy and intentional than most. His main defense was that this was business as usual, the kind of thing all politicians do — pure cynicism ingrained to a shocking degree.

Yet the judges’ thinking is similar. Citing the Skilling and McDonnell rulings as precedent, they decided that even with the architect of a bribery empire, busted wide open, can’t be found guilty without the jury being brainwashed by the court’s intentional loosening of corruption laws.

Unconscionable.

Thankfully, the decision opened the possibility of a retrial, which we hope will be swift and decisive in once again convicting Silver and sending him to prison for a long time — maybe the rest of his life. But why should it take such an extraordinary effort to convict one of the most blatant political crooks?

Considering a terrible fallout from the Skilling decision, we cringe to think about the precedent the Silver ruling sets. As former public corruption prosecutor Peter Zeidenberg told the New York Times, “This, indeed, may be the beginning of a parade of horribles.”

If judges find this much wiggle room in existing laws, it’s clear that lawmakers must pass new, tougher ones. We doubt they will do so without a fight, though; many of the most powerful legislators are probably rejoicing at the Silver ruling. Therefore, the people will have to demand it, and vote them out otherwise.