To lock up voters’ support on Election Day, a politician can spend tens of millions, even billions, of public dollars on a program, entitlement, or tax break that is guaranteed to be popular with a key demographic. A $25,000 subsidy for first-time home buyers? A tax break for tipped workers? A massive scheme to “forgive” student loan debt? In order to win elections, candidates routinely promise to deliver such costly benefits, assuring voters they will be financially better off if they back the right candidate.

As public policy, such promises are often indefensible. They may even be unconstitutional. As a campaign tactic, however, they are perfectly lawful. And yet a candidate who offers to pay even a negligible amount of money directly to individuals in exchange for their votes is committing a crime and can be prosecuted and imprisoned. In the United States, as Stanford law professor Pamela Karlan once wryly observed in a law review article, politicians are only allowed to buy votes wholesale. Retail vote-buying is illegal.

Why is that? Is it really so bad to trade your vote for cash or some other gratuity?

For much of American history, vote-buying was routine, if frowned on. Until the last years of the 19th century, voting took place in the open and voters could show up with filled-out ballots — helpfully peddled by political parties or printed in partisan newspapers — to drop in the ballot box. Under those circumstances, it was easy to find voters who would gladly swap their vote for a drink or some money.

With the advent of the secret ballot — Massachusetts led the way in requiring private voting booths and blank ballots supplied by the state — vote-buying became much harder. Still, though every state and the federal government banned the practice, it persisted in parts of the country.

In his magisterial biography of Lyndon B. Johnson, historian Robert Caro describes how in 1934, as an operative for a Texas congressional campaign, LBJ “sat in a San Antonio hotel room behind a table covered with five-dollar bills, handing them to Mexican-American men at the rate of five dollars a vote for each vote in their family.” A Time magazine story in 1960 described the “half-pint vote” as an ongoing practice in southern West Virginia, where the “standard payoff” for someone’s vote was a “half-pint of bourbon whisky and $2 to $5 in cash.” In 2012, the Justice Department successfully prosecuted two Arkansas men for a scheme to buy the votes of residents who had requested absentee ballots.

Are such cases of vote-buying reprehensible? Most of us, I imagine, share an instinctive sense that to directly trade votes for money is sleazy and outrageous. But it’s harder than you might think to convincingly explain why.

Harvard economist Greg Mankiw grappled with the question when it was posed to him by his colleague, the philosopher Michael Sandel, during a session of Sandel’s renowned course on justice.

“If you economists are so in favor of voluntary exchange,” challenged Sandel, “would you extend that conclusion to letting a person sell his right to vote to another person?”

No, said Mankiw. Even though both parties to the voluntary exchange would be better off, vote-selling must be prohibited because of “externalities.” That is economist-speak for the impact that a transaction may have on unrelated third parties. When Alex agrees to vote for Bob in exchange for (say) $25, he is also affecting the future of Clara, who may be forced to endure policies supported by Bob that she opposes.

But wait. Suppose Alex agrees to vote for Bob not because he is paid to do so but because Bob persuades him to do so in a conversation. Or charms him into doing so through flattery. Or badgers him into doing so by bending his ear until he agrees. Or deceives him into doing so by lying about what his opponent will do if he wins. In all those cases, no payment is involved but the outcome is the same: Alex casts a vote for Bob, potentially harming Clara’s future.

If all those means of winning a vote are permitted, why not a straightforward payment?

Karlan, the Stanford law professor, suggests other explanations. The gaudy benefits promised by candidates don’t actually become real until the public chooses to elect the candidate, while the naked purchase of a vote with cash cuts the public out of the process altogether. In that sense, vote-buying turns the whole point of an election on its head.

Conversely, once voters are paid to mark a ballot for a certain candidate, they have no future claim on that candidate. “If Candidate A sees herself as having discharged her responsibility to Voter X by paying him,” Karlan wrote in her law review essay, “this may undermine any sense of her continuing duty to represent him.”

Some scholars propose an equity rationale for banning direct vote-buying: Poor people are more likely to sell their votes, so elections will be skewed in favor of the wealthy. Others contend that votes are not personal property; they belong to the body politic and cannot be used by individuals for their private enrichment.

Those are all ingenious arguments, but does any of them really clarify why trading votes for money is so widely regarded as scandalous? My own view is that vote-buying feels to most of us like a desecration of something sacred. However debauched our politics have become, the act of voting still evokes a feeling of reverence — a feeling that we are participating in the gravely eloquent ritual on which our system of self-governance turns.

Once voting became a secret act, it took on a numinous quality. In the final hours of his 1992 presidential campaign, Bill Clinton spoke of the “great mystery of American democracy.” Perhaps it was to maintain that sense of mystery that direct vote-buying — which by the 1880s had become thoroughly associated with corrupt political machines — became so anathematized.

Until recently, all this might have been of merely theoretical interest. But two modern developments have once again made it extremely easy to pay people for their guaranteed vote. One is the ubiquity of cellphone cameras and texting; it’s a cinch to send proof of a vote from inside the voting booth. The other is the post-pandemic acceptance of easy absentee voting through the mail or public drop boxes. A voter has only to fill out a ballot, then hand it to the campaign worker to be sealed in the envelope and dropped in the mail.

In an age when so many other norms have fallen by the wayside, will this norm continue to be protected? For more than a century, it has helped imbue America’s often raucous system of self-government with an element of the sacrosanct. With all due respect to the philosophers and economists, that is reason enough to preserve it.