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 — doing X in exchange for Y — had occurred because McDonnell arranged some meetings between the businessman and some state officials.

McDonnell appealed to the Supreme Court, arguing that legal precedents say that bribery occurs only when “official acts” are done in response to the receipt of something valuable, and that what he did — arranging contacts for the businessman, attending receptions with him, etc. — were not exercises of government power, hence were not official acts.

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.”

In a democracy, politics is always and everywhere transactional: Voters support a candidate in the expectation that they will be rewarded with certain policies, and politicians promise to benefit supporters with particular 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.

“The word ‘influence’ is too broad, because every day of the week politicians write on behalf of constituents letters to different parts of government.”

Furthermore, Breyer says 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.”

Breyer’s anxiety suggests that the court’s liberals, too, are increasingly interested in the threat the administrative state’s rampant executive branch poses to checks and balances.

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 favor incumbents over challengers or certain kinds of supporters over others.

In 2014, 54 members of the Senate Democratic Caucus tried to do something never done in the 225 years since the Bill of Rights was ratified: They tried to amend it to make it less protective of liberty.

They sought to amend the First Amendment in order to empower government to restrict the spending necessary for the dissemination of campaign speech about the government’s composition and comportment.

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, so government regulation of politics is presumptively salutary.

These moral certitudes are more menacing to civic health than were 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.

George Will is a columnist with The Washington Post Writers Group.