The oil giants ExxonMobil and Chevron each have assets valued in the hundreds of billions of dollars. Last year, The Wall Street Journal recently revealed, the two companies considered what would have been among the largest corporate mergers in history-a deal that would have reunited parts of the Standard Oil empire that federal trustbusters broke apart in 1911.

In the end, ExxonMobil and Chevron didn’t attempt the transaction. But had the companies insisted on it, today’s antitrust authorities probably would have permitted the tie-up. Mergers among the very largest corporations are rarely stopped.

Our research found that, out of the 78 proposed mergers from 2015 to 2019 in which the smaller firm was valued at more than $10 billion, the federal government attempted to block a grand total of only five on antitrust grounds and successfully stopped just three of them.

In February 2020, a district judge allowed T-Mobile (with a premerger equity valuation of more than $50 billion) to acquire Sprint for $30 billion and gave control of the national wireless market to just three carriers.

As evidence mounts that corporate consolidation and concentration raise prices to consumers, eliminate jobs, depress wages, marginalize independent businesses, and breed economic and political inequality, Democrats in Congress, possibly in collaboration with some Republican colleagues, appear poised to crack down on monopoly and prevent further consolidation.

At the top of this agenda should be a law that simply and unambiguously prevents all megamergers—which we would define as transactions in which the acquirer and the target each has more than $10 billion in assets.

Such a rule would have stopped dozens of mergers that were completed in the second half of the 2010s, including the acquisitions of SABMiller by Anheuser-Busch InBev, Aetna by CVS, and Monsanto by Bayer. In general, corporate consolidation does not improve business productivity.

Melissa Schilling, a business professor at New York University, has concluded that “most mergers do not create value for anyone, except perhaps the investment bankers who negotiated the deal.” Those findings make the government’s willingness to rubber-stamp so many recent mergers all the more remarkable.

The Congresses that enacted the nation’s antitrust laws understood that unchecked corporate power makes a mockery of democratic norms. In 1890, Senator John Sherman, an Ohio Republican, helped develop the nation’s first federal antitrust act in response to the rise of corporate and financial titans such as J.P. Morgan.

Sherman insisted that the country’s economic life should not be dominated by “a few men sitting at their council board in the city of New York.” In a 1958 decision, the Supreme Court echoed this theme, stating that “the Sherman Act was designed to be a comprehensive charter of economic liberty” that aimed to provide “an environment conducive to the preservation of our democratic political and social institutions.”

Sadly, that tradition gave way in the 1970s and ’80s, as federal judges, the Justice Department’s antitrust division, and the Federal Trade Commission all came under the spell of dubious interpretations of history and economic theories strikingly tolerant of mergers and monopolistic practices.

Without strong evidence that mergers will raise consumer prices and reduce economic output, federal antitrust agencies and courts hesitate to act even against companies that dominate their market. For the Justice Department, the FTC, and courts reviewing merger matters, considerations of political power, including the absolute size of the corporations involved, are irrelevant.

The history of consolidation in the oil industry is revealing and suggests that an ExxonMobil-Chevron merger is not far-fetched. In the late 1990s and early 2000s, the FTC permitted very large oil and gas corporations to merge on the condition that they sold off gas stations, refineries, and other assets to “preserve competition” in markets where they were head-to-head competitors or in a position to exclude rivals.

The tolerance of mergers has spread corporate concentration and its attendant inequality into virtually every corner of the economy: health care, airlines, cable TV, and now the internet, where Amazon, Facebook, and other sprawling new monopolists reign.

A small clique of executives and financiers makes key decisions in our economy. Many figures across the political spectrum are now urging a return to the kind of antitrust enforcement that once helped preserve a variety of independent businesses in every community.

Among these voices, for example, is Senator Elizabeth Warren, who called for tight merger restrictions for companies that have more than $40 billion in annual revenues. In a fall 2019 presidential-candidate debate, she said: “We need to enforce our antitrust laws, break up these giant companies that are dominating Big Tech, Big Pharma, Big Oil, all of them.” Earlier this month, Senator Amy Klobuchar, together with four co-sponsors, proposed including a corporation’s absolute size in merger analysis. In October 2018, Senator Bernie Sanders introduced a bill that would break up the largest financial institutions in the United States and establish a cap on size going forward.

Although conservatives in the United States have generally supported Big Business interests, more voices on the right are grafting concerns about corporate power, particularly in digital markets, onto an otherwise standard right-wing agenda.

Although former President Donald Trump’s administration had a poor antitrust record against large corporations and supported pro-monopoly reinterpretations of the law, it did file landmark suits against Google and Facebook in the closing months of 2020. Embracing some forms of economic populism, media outlets such as The American Conservative have also become supporters of renewed antitrust enforcement.

Building on ideologically diverse opposition to corporate consolidation, Congress should pass legislation that strikes at mergers, a major contributor to the curse of corporate bigness. A ban on mergers involving companies that have more than $10 billion in assets might be a somewhat arbitrary line to draw-Congress could reasonably choose a higher or lower threshold-but the formulation and administration of law, which establishes the rules of a market, requires a degree of line-drawing.

Anyway, the status quo, in which virtually every merger goes forward, almost regardless of the potential damage to customers, suppliers, rivals, workers, and even democracy, is arbitrary in its own way and runs contrary to the public interest.

Under the legislation we propose, a future merger between Chevron and ExxonMobil would be plainly illegal. Even if they agreed to sell some assets to a third party—as many merging companies do—the two oil titans would not be able to get their transaction past the antitrust authorities. The companies probably would not even contemplate such a combination in the boardroom.

By establishing a bright line, an outright ban on the largest mergers would reduce the role of contending lobbyists, lawyers, and rented economists in merger cases, thereby making the whole process clearer, faster, more predictable, less expensive, and less subjective, as we explain at greater length in a recent law-review article.

A ban on megamergers would reduce the amount of money and human energy currently wasted in putting together unproductive consolidations. It would help end the arms race of consolidation, in which mergers beget mergers as firms try to keep up with ever larger and more powerful corporate rivals, suppliers, and customers. By potentially channeling these resources into new productive capacity and technologies, the law could result in a real increase in society’s overall wealth and pace of progress.