WASHINGTON – The progressive drive to broadly define and thoroughly eradicate political “corruption” has corrupted politics. But discord is not altogether pandemic in Washington, and last week a unanimous Supreme Court, in this term’s most important decision, limited the discretion prosecutors have to criminalize politics.

Former Virginia Gov. Bob McDonnell was sentenced to prison for unseemly behavior. He accepted from a Virginia businessman gifts and loans valued at more than $170,000. The businessman wanted McDonnell to help promote his dietary supplement business, including by helping him persuade state universities to study its products.

The businessman did not get his money’s worth: No government action was ever taken on his behalf.

Prosecutors, however, convinced a jury that quid pro quo corruption had occurred because McDonnell arranged some meetings between the businessman and some state officials. McDonnell appealed to the Supreme Court, arguing legal precedents say bribery occurs only when “official acts” are done in response to the receipt of something valuable, and that what he did were not exercises of government power.

Searching, like Flaubert, for le seul mot juste, Chief Justice John Roberts offered “tawdry” and “distasteful” to describe McDonnell’s behavior. But neither adjective is a synonym for “criminal.” Besides, the gifts then were then legal under Virginia law.

In a democracy, politics is always and everywhere transactional: Voters support a candidate in the expectation they will be rewarded with certain policies. The court has held that politicians granting “access” to supporters who have made acts of “ingratiation” is not corruption.

During oral arguments in April, Justice Stephen Breyer issued a warning that was echoed by Roberts’ opinion last week. Breyer said a definition of corruption that includes a government official like McDonnell trying to “influence” government actions “puts at risk behavior that is common.”

Furthermore, Breyer said prosecutors’ unchecked power to define corruption poses “as fundamental a real separation of powers problem as I’ve seen” because “the Department of Justice in the executive branch becomes the ultimate arbiter of how public officials are behaving in the United States.”

As political puritans pursue ever-more-perfect civic hygiene, and progressives pursue ever-more-comprehensive government, the crusade against “corruption” expands to cover a multitude of sins. Campaign finance regulations, ostensibly enacted to combat corruption or the “appearance” thereof, become corruption: They become weapons written for self-dealing incumbents or for partisan advantages.

So progressives, who want government control of almost everything, justify its control of the quantity and timing of campaign speech by arguing this: The transactional nature of democratic politics is inherently unseemly and campaign contributions are incipient corruption (here, too, Donald Trump agrees with progressives).

These moral certitudes are more menacing to civic health than was McDonnell’s indefensible but not criminal comportment. And the multiplicity of campaign regulations means the court has more to do regarding the deregulation of politics.