Former Virginia governor Robert McDonnell won big on Sept. 8 when the corruption charges he had been fighting for four years were dropped. Here's what you need to know. (Monica Akhtar/The Washington Post)

No one thinks Bob McDonnell and his wife, Maureen, acted nobly in their dealing with dietary supplement businessman Jonnie Williams. They accepted $175,000 in gifts and donations — Rolexes! Ferrari rides! Wedding gifts! — from Williams, who was relentlessly trying to gain access to the Virginia governor and his wife in hopes of improving the chances of his various businesses succeeding.

And yet, the Supreme Court unanimously overturned McDonnell's conviction on corruption charges Monday. Why? Because it is very, very hard to convict politicians on public corruption charges without a smoking gun. And there wasn't one in this case.

"There is no doubt that this case is distasteful; it may be worse than that," wrote Chief Justice John G. Roberts Jr. in the majority opinion. "But our concern is not with tawdry tales of Ferraris, Rolexes, and ball gowns. It is instead with the broader legal implications of the government’s boundless interpretation of the federal bribery statute.”

What Roberts is saying is this: Yes, McDonnell and his wife breached the public trust and drastically underperformed the expectations we have for our elected officials. But disappointment and tawdriness aren't illegal.

Again, Roberts:  “If the court below determines that there is sufficient evidence for a jury to convict governor McDonnell of committing or agreeing to commit an ‘official act,’ his case may be set for a new trial. If the court instead determines that the evidence is insufficient, the charges against him must be dismissed. We express no view on that question.”

Sure, McDonnell and his wife accepted all sorts of gifts from Williams. And sure, Williams was trying to curry favor from McDonnell to help the dietary supplement he was hawking. But Roberts is asking, what is the thing that McDonnell did in exchange for all of those gifts that would make this an obvious example of selling his office to the highest bidder?

McDonnell had always insisted that there was no quid pro quo tied to Williams's donations, arguing that the things Williams allegedly bought with his largesse — a lunch held at the governor's mansion to benefit his product, contacts shared by McDonnell with Williams, etc. — were in keeping with what he as governor would do for any Virginia-based company or entrepreneur. (McDonnell broke no law in his willingness to accept these gifts, either. His successor, Terry McAuliffe, significantly tightened the gift limits in reaction to outcry over McDonnell.)

“Setting up a meeting, calling another public official, or hosting an event does not, standing alone, qualify as an ‘official act,’ " wrote Roberts.

The court's ruling (and its unanimous nature) affirms just how difficult it is to convict a politician of selling his or her office — unless that politician is either remarkably stupid or remarkably blatant in flouting the law. Unless you have somewhere in writing "Please give X person this official thing in exchange for a gift," it's very tough to prove that the reason a politician was motivated to help someone was because of a gift to them.

It's why allegations of a politician voting a certain way in exchange for campaign contributions from an individual or group of allied interests almost never works. How can you prove that the politician wasn't acting in what he believed was the best interests of his state and not at the behest of campaign donors? You can't — unless, as I noted above, the person is a giant moron and says that on tape or in writing.

This ruling — depending on how the lower courts decide to deal with the Supreme Court's decision — might keep McDonnell from ever serving a day in jail. (He was allowed to remain out of the pokey while his appeal was moving through the courts.) But it won't save his political career, which has been permanently damaged by this whole saga. (Sidebar: Proof, yet again, that guilt in the court of public opinion is a very different thing than guilt in a court of law.)

What the ruling will do is have a potentially profound effect on future cases involving allegations of public corruption against politicians. This tweet by Washington Post congressional reporter Mike DeBonis gets at those implications for New Jersey Democratic Sen. Bob Menendez:

Menendez was indicted on federal corruption charges in April 2015, based on allegations that he traded the power of his office for more than $750,000 in campaign donations and other gifts from a Florida-based eye doctor named Salomon Melgen. Menendez has long insisted that Melgen is longtime personal friend and that any connection between official actions on Melgen's behalf and the contributions is entirely coincidental.

If Menendez or Melgen (or anyone close to them) never put anything in writing or on tape that directly connects donations or gifts to actions by the senator, it could prove extremely difficult — in light of Monday's McDonnell ruling — to prove that the New Jersey Democrat was doing anything other than what he thought was right.

No court — not even the Supreme one — can crawl inside a politician's head to see what he was thinking at any given moment. That makes determining intent — unless a politician documents his intent — virtually impossible. That's what the Supreme Court told us today,  and why McDonnell and Menendez are each having their best Monday in quite some time.

The Post's Robert Barnes explains the big decisions in this Supreme Court session and how the absence of a justice will continue to affect it. (Gillian Brockell/The Washington Post)