'Are gentlemen in earnest?" James Madison challenged his fellow delegates to the Constitutional Convention in the summer of 1787. "Are we not struck at seeing the luxury and venality which has already crept in among us?" How staggered might Madison be to see the activities that are now considered "business as usual" in Washington a little more than two centuries later, not least the preponderant role of fundraising and lobbyists in the life of public officials. And at last someone has written a book that puts a name to what is perhaps the most significant factor shaping American politics today: corruption.

In a masterly work of scholarship, Zephyr Teachout, a Fordham Law School professor who recently ran in the New York Democratic gubernatorial primary, traces the history of American approaches to what was long considered a mortal threat to the republic. She demonstrates that recent jurisprudence, which has whittled down the definition of corruption to encompass only a contractual exchange between briber and public official, represents nothing less than "a revolution in political theory."

"Corruption in America" opens with the architects of American democracy, who were vehement about public integrity. "If we do not provide against corruption," Virginian George Mason warned fellow delegates to the Constitutional Convention, "our government will soon be at an end." Fears about the ability of corruption to thwart the democratic process pervaded the Founders' discussions. Debates over fundamentals—the relative size of the two houses of Congress, for example, or the length of terms in office—were all, in fact, struggles over how to protect public officials from the potentially corrupting influence of private interests. During the convention, Mason used the British example to argue against allowing the president to appoint key officials: "By the sole power of appointing the increased officers of government, corruption pervades every town and village in the kingdom." James Madison, the author notes, feared both houses of Congress "were likely to engage in 'schemes of usurpation or perfidy'—without a check, each on the other, a single body could come under the sway of 'ambition or corruption' and government would betray the people." Ms. Teachout sums up: "The word corruption was used hundreds of times in the convention and the ratification debates." And yet "only a handful of uses referred to what we might now think of as quid pro quo bribes."

She dwells on the controversy over French King Louis XVI's 1785 gift of a diamond-studded snuffbox inset with the king's portrait to departing U.S. Ambassador Benjamin Franklin. The public outrage over the treasure helps to illustrate that, as Ms. Teachout notes, a gift to a public official did not have to be bestowed with "corrupt intent" to be considered a threat to the official's independence. For the Founders—and for many later generations of American leaders—just the perception of behavior that might be conducive to corrupt practices warranted robust preventive action.

When delegates met to draft the Constitution a year or so later, that royal snuffbox helped prompt the remarkably strict prohibition against foreign gifts they adopted: "No Person holding any Office of Profit or Trust . . . shall, without the Consent of the Congress, accept of any present, Emolument, Office, or Title, of any kind whatever, from any King, Prince, or foreign State." In the end, Franklin's colleagues let him keep the snuffbox. But the ban, in a puritanical break from prevailing custom in Europe, was inscribed in the Constitution with virtually no debate.

Critically, the ban was not limited to gifts that were provided with the expectation of a specific action in return. It was a blanket prohibition, for "we expect that gifts lead to some warmth and generosity toward the giver," as Ms. Teachout puts it, and these feelings could "interfere with [the recipient's] responsibility to put the country's interest first." The Framers, she writes, sought to move the United States away from the "culture of gift-giving." The prohibition on foreign gifts has survived like a fossilized artifact in Washington—the modern equivalent of a valuable snuffbox (any object worth more than $305) must be turned over to the recipient's agency and purchased back at market value if he or she wishes to keep it. And yet, gifts from domestic individuals or corporations vying for influence are commonplace. The gift economy now pervades U.S. politics.

The recent corruption trial of former Virginia Gov. Robert McDonnell illustrates the dramatic shift in public morals that "Corruption in America" charts. The loans and luxury gifts that McDonnell and his wife received are not today considered wrong in and of themselves. The prosecution had to establish a direct connection—a quid pro quo—between these favors and the actions that McDonnell took on behalf of his benefactor, businessman Jonnie Williams. "Bob never gave Jonnie anything special" in return for the largess, the defense contended, arguing that when McDonnell set up meetings for Mr. Williams, or hosted events for him at the governor's mansion, he was showing the same courtesy he might have extended to any Virginia businessman. "This case is all quid, no quo," insisted the defense attorney. (The jury disagreed.) But until just a few decades ago, Ms. Teachout argues, such gratuitously handsome "quids" would have been anathema, whether or not the politician had helped Mr. Williams in return.

Aside from the direct gift-giving with which the Framers were familiar, the American republic developed new practices conducive to influence peddling, such as the use of intermediaries, or lobbyists, to approach public officials. Ms. Teachout points out that paid lobbying, a practice taken for granted today, was either prohibited or ruled contractually void in state constitutions and case law going back to the 1850s. As the Supreme Court's 1874 opinion in Trist v. Child put it: "If any of the great corporations of the country were to hire adventurers who make market of themselves in this way, to procure the passage of a general law with a view to the promotion of their private interests, the moral sense of every right-minded man would instinctively denounce the employer and the employed as steeped in corruption, and the employment as infamous."

Gradually views of lobbying shifted. First, it was reclassified as a professional service, not the sale of personal influence. Beginning in 1890, new state lobbying-registration laws established the professional status of lobbying services even when performed by nonlawyers. In 1927, courts began enforcing lobbying contracts. In 1946, two Supreme Court cases gave tacit approval to a defense of lobbying as free speech under the First Amendment—though, as Ms. Teachout notes, "they provide no guidance on the scope of that right." In the decades since, as she chronicles, the Supreme Court has broadened the definition of protected political activities, increasingly and directly equating money with speech.

The first case that explicitly established that equation came in 1976. In Buckley v. Valeo, the Supreme Court struck down on free-speech grounds the part of a post-Watergate reform that limited the amount of money that could be spent in political campaigns. Before Buckley, writes Ms. Teachout, "both campaign expenditure limits and contribution limits were presumptively valid." After the decision, however, "every state or federal law involving a regulation of money and politics became suspect and open to challenge." This, she argues, made the development of the modern gift economy inevitable.

In McCormick v. United States (1991), the court overturned West Virginia Delegate Robert McCormick's conviction for soliciting contributions in return for sponsoring a bill because the payments were not "made in return for an 'explicit promise' to do (or not do) an official act." This formulation, Ms. Teachout writes, laid the groundwork for today's cramped definition of corruption. Finally, Citizens United v. Federal Election Commission (2010) found that unions and corporations enjoy the same constitutionally protected free-speech rights as individuals and thus can spend an unlimited amount on national elections. "The opinion comprehensively redefined corruption," writes Ms. Teachout. "As a matter of federal constitutional law, corruption now means only 'quid pro quo' corruption.' And quid pro quo exists only when there are 'direct examples of votes being exchanged for . . . expenditures.' " She concludes: "The corruption against which the framers said they must provide, lest 'our government will soon be at an end'—that is not a value anymore."

What is the remedy? Ms. Teachout calls for a return to the Framers' preference for across-the-board rules to help prevent corrupt acts before they are perpetrated, rather than relying on punishment after the fact. Under the current system, in order for actual corruption to be proved, intent must be established—always a difficult feat—and, above all, corruption must be defined. If that definition is too narrow, then the resulting laws and punishments will be so divorced from the realities of the political economy as to have little deterrent effect. "If a bribery statute is narrowly drawn (or interpreted)," Ms. Teachout writes, "it covers only brazen, unsophisticated exchanges and does not actually solve problems of money being used to influence policy and undermine representative government."

In the wake of the McDonnell conviction, Virginia is considering such a structural rule: a ban on all gifts to public officials above a certain value. New York City's 6-1 public match for individual campaign contributions aims to put ordinary residents on a more equal footing with corporations or out-of-state donors. Beyond specific policies, however, Ms. Teachout argues that citizens need to revive the expectation that public officials place the public good ahead of private interests. She calls this the "anticorruption principle" and makes a powerful case that all of us must champion it if we want our democracy to survive.

This book review was originally published in the Wall Street Journal.