THIS week, the Supreme Court heard McDonnell v. United States, the case of Bob McDonnell, the former governor of Virginia who is appealing his 2014 conviction for public corruption. Although the court’s ruling is not expected until June, in Wednesday’s hearing several justices seemed set on undermining a central, longstanding federal bribery principle: that officials should not accept cash or gifts in exchange for giving special treatment to a constituent.

Justice Stephen G. Breyer dismissed the idea that, in the absence of a strong limiting principle, federal law could criminalize a governor who accepted a private constituent’s payment in exchange for intervening with a constituent problem. Justice Samuel A. Alito Jr. expressed disbelief that an official requesting agency action on behalf of a big donor would be a problem. A majority seemed ready to defend pay-to-play as a fundamental feature of our constitutional system of government.

In September 2014, after a six-week trial, a federal jury convicted Mr. McDonnell and his wife, Maureen, on multiple counts of extortion under the Hobbs Act, a key statute against political corruption, and honest-services fraud. It was not a complicated case. Jonnie R. Williams Sr., the chief executive of a dietary supplement manufacturer, Star Scientific, had showered the governor and first lady with gifts in return for favors.

We’re not talking about a few ham sandwiches. The McDonnells took expensive vacations, a Rolex, a $20,000 shopping spree, $15,000 in catering expenses for a daughter’s wedding and tens of thousands of dollars in private loans. In exchange, the governor eagerly promoted Mr. Williams’s product, a supplement called Anatabloc: hosting an event at the governor’s mansion, passing out samples and encouraging universities to do research.