Suppose you hope to land a big government contract. Here's one way you might go about getting it: You hire a lobbyist, who just happens to be a longtime friend of the governor. He hands the governor $10,000 in cash, maybe in a briefcase, like in the movies. The governor then picks up the phone, calls the head of the agency awarding the contract and you're in, ready to make your pitch.

Maybe that sounds like a corrupt arrangement. But that scenario, suggested by Randall Eliason, a George Washington University law professor, would be perfectly legal under current interpretations of ethics laws.

The recent mistrial and dismissal of charges against U.S. Sen. Robert Menendez made it clear that juries don't consider gifts between friends, no matter how valuable, to be proof of corruption. More importantly, the U.S. Supreme Court's 2016 ruling in a case involving former Virginia Gov. Bob McDonnell has substantially increased the burden on prosecutors.

Since the McDonnell ruling, a number of high-profile corruption convictions have been thrown out -- notably those involving Dean Skelos, the former New York Senate majority leader; Sheldon Silver, the former New York Assembly speaker; and William Jefferson, a former Louisiana congressman who stored $90,000 worth of bribe money in his office freezer. 

Silver's new trial is scheduled to start on April 16. Skelos will be tried again in June. But prosecutors face a different task than they did during the original trials.

Prior to the McDonnell verdict, prosecutors could suggest, through circumstantial evidence, that an illegal quid pro quo had taken place. Now they have to show a much closer connection between moneys paid and favors made.

"From the outside, it looks like the government has to come close to proving an explicit quid pro quo," says Jonathan Jeffress, a white collar defense attorney in Washington. "If they don't have that, their evidence may come up short. That wasn't the case before."

McDonnell accepted more than $175,000 in cash and prizes from Johnnie Williams, the owner of a dietary supplement company looking to do business with the state. McDonnell set up meetings and let Williams host a luncheon at the governor's mansion to launch a product.

But the Supreme Court vacated McDonnell's convictions, finding that while the governor might have helped Williams out, nothing he did constituted an "official act." Setting up meetings with subordinates doesn't count as an official act in the way that, say, a vote in the legislature would, or ordering an underling to make a decision.

Prosecutors looking to make corruption cases now have to prove that an official took some action that meets a strict definition of an "official act" in exchange for something of value. It seems like you'd almost have to go out of your way to get caught doing something explicitly, when you could make your desires clear to subordinates through unprosecutable hints.

"It's just going to be all understood rather than stated," Jeffress says. "It's a wink and a nod."

Prosecutors stress that public officials haven't been given a green light for corruption. A big question going forward, however, is how close in time payments have to be made in exchange for official acts. Rather than handing over a sack of money when services are rendered, someone bribing an official might dole out gifts and payments over a period of months or years, essentially keeping the official on a kind of retainer. 

Lower courts have accepted this "stream of benefits" theory and the judge in the Menendez trial explicitly said it's still valid, but the Supreme Court has never weighed in.

"What we're going to see over the next few years is the extent to which it's going to remain after McDonnell," says Daniel Richman, a Columbia University law professor. "Whether it does or not will make a big difference in how we assess McDonnell's effects."

In its McDonnell ruling, the Supreme Court expressed concern about public officials having to worry about possible prosecution every time they help a friend or a donor. In a world where politicians routinely pick up the phone on behalf of campaign contributors, where's the line between that and accepting a cash gift (particularly in states where politicians can put campaign funds to personal use)? Trading favors is part of the currency of politics. 

All this means that a longstanding dynamic in politics now seems truer than ever: The scandal is what's legal, not what's illegal.