Hoping that federal prosecutor

Keep hoping. The Supreme Court could make it harder to secure corruption convictions — meaning we’ll have to kick dirty pols out at the ballot box, rather than wait for arrests.

Federal law is clear: A politician can’t take a donor’s money and promise something in return.

Bill de Blasio can’t take money from people who want to ban the horse carriages — and promise to ban the horse carriages in return.

Nor can he take money from people who don’t want to ban helicopter tours — and then protect the tours in return.

So why might the mayor skate when it comes to any pay-for-play shenanigans?

The key is the “in return” part.

The Supreme Court has long prohibited prosecutors from using timing as evidence: Jurors must believe it is just coincidence if a candidate gets a donation one day and does something for the donor the next.

Prosecutors need evidence of an explicit deal, and few politicians are dumb enough to have such a deal.

Now, the Supreme Court could make it even harder for prosecutors to prove a quid pro quo. Under a possible new reading of the law, politicians would be able to make explicit deals with donors — as long as they don’t act “officially.”

The case in question is federal prosecutors’ conviction two years ago of then-Virginia Gov. Bob McDonnell.

McDonnell received a prison sentence for taking $177,000 in plane trips, Louis Vuitton and Oscar de la Renta clothes, iPhones, a wedding for his daughter and a Rolex, all from a man, Jonnie Williams, who wanted the governor to promote his diet pill.

The jury was careful — it acquitted the governor on three counts. But jurors could see the obvious: No politician can take that much money and not know he is being bribed.

Still, McDonnell has a great lawyer, Noel Francisco — who convinced the Supreme Court to take the case.

Last Wednesday, the court heard a novel argument: McDonnell can’t be convicted of bribery, because the things he did for Williams were not “official acts.”

That is, when McDonnell asked researchers at the state University of Virginia to promote Williams’ drug supplements, he wasn’t acting as governor. He was just acting as any regular person, exercising free speech.

As Justice Elena Kagan observed of one of the governor’s techniques, “so that’s essentially hosting a party and allowing Mr. Williams to invite some people . . . Why is that an official act?”

Sure. We all have parties in our homes.

Except: The governor has parties in the Governor’s Mansion — not something anyone can do.

And, as Justice Department lawyer Michael Dreeben noted before the court, the guests from the university weren’t exactly there voluntarily.

The governor appoints the university’s board members. “He sets the budget,” Dreeben noted. “They know that he’s an important guy . . . The governor is taking every step he can” to signal to university folk to support this diet drug.

When you’re the governor, everything you do in public is an official act.

If corruption is only when you take direct action to help a donor, it will be difficult to secure corruption convictions.

A change in the law would affect New York’s recent cases — and chances for future convictions.

Take former state Senate leader Dean Skelos, convicted of bribery in December. US Attorney Bharara’s conviction of Skelos wasn’t mostly based on “official acts,” under the definition the court is considering.

Skelos convinced Long Island officials to help a company that employed his son, in return for that employment. But Skelos had no legal authority over those Long Island officials. Anyone can ask government officials for help.

The same is true with the mayor. Say prosecutors do find evidence of a deal between horse-carriage opponents and de Blasio. Any citizen can propose that the City Council consider a bill to ban horse carriages — just as any citizen can encourage regulators to look favorably upon helicopters.

(Silver’s conviction is more secure. Then again, it took two decades to catch the guy.)

No matter what the Supreme Court rules, we shouldn’t rely on the justice system to govern our politicians.

De Blasio doesn’t have to be found legally corrupt for the voters to think he is corrupt.

As long as he’s out of office, it doesn’t matter if he’s out of prison, too.

Nicole Gelinas is a contributing editor to the Manhattan Institute’s City Journal.