It would be understandable if you assumed that the Anti-Merger Act of 1950 had been repealed. But in fact it remains on the books. It has merely been evaded, eroded and enfeebled by the corroding effect of decades of industry pressure and ideological drift, yielding hesitant enforcers and a hostile judiciary. Consequently, over the last two decades we have allowed successive waves of mergers that make a mockery of the 1950 law, and have concentrated economic power in ways that are dangerous to the polity.

In recent years, we have allowed unhealthy consolidations of hospitals and the pharmaceutical industry; accepted an extraordinarily concentrated banking industry, despite its repeated misfeasance; failed to prevent firms like Facebook from buying up their most effective competitors; allowed AT&T to reconsolidate after a well-deserved breakup in the 1980s; and the list goes on. Over the last two decades, more than 75 percent of United States industries have experienced an increase in concentration, while United States public markets have lost almost 50 percent of their publicly traded firms.

There is a direct link between concentration and the distortion of democratic process. As any undergraduate political science major could tell you, the more concentrated an industry — the fewer members it has — the easier it is to cooperate to achieve its political goals. A group like the middle class is hopelessly disorganized and has limited influence in Congress. But concentrated industries, like the pharmaceutical industry, find it easy to organize to take from the public for their own benefit. Consider the law preventing Medicare from negotiating for lower drug prices: That particular lobbying project cost the industry more than $100 million — but it returns some $15 billion a year in higher payments for its products.

We need to figure out how the classic antidote to bigness — the antitrust and other antimonopoly laws — might be recovered and updated to address the specific challenges of our time. For a start, Congress should pass a new Anti-Merger Act reasserting that it meant what it said in 1950, and create new levels of scrutiny for mega-mergers like the proposed union of T-Mobile and Sprint.

But we also need judges who better understand the political as well as economic goals of antitrust. We need prosecutors willing to bring big cases with the courage of trustbusters like Theodore Roosevelt, who brought to heel the empires of J.P. Morgan and John D. Rockefeller, and with the economic sophistication of the men and women who challenged AT&T and Microsoft in the 1980s and 1990s. Europe needs to do its part as well, blocking more mergers, especially those like Bayer’s recent acquisition of Monsanto that threaten to put entire global industries in just a few hands.

The United States seems to constantly forget its own traditions, to forget what this country at its best stands for. We forget that America pioneered a kind of law — antitrust — that in the words of Roosevelt would “teach the masters of the biggest corporations in the land that they were not, and would not be permitted to regard themselves as, above the law.” We have forgotten that antitrust law had more than an economic goal, that it was meant fundamentally as a kind of constitutional safeguard, a check against the political dangers of unaccountable private power.

As the lawyer and consumer advocate Robert Pitofsky warned in 1979, we must not forget the economic origins of totalitarianism, that “massively concentrated economic power, or state intervention induced by that level of concentration, is incompatible with liberal, constitutional democracy.”

Tim Wu (@superwuster) is a law professor at Columbia, a contributing opinion writer and the author of the forthcoming book “The Curse of Bigness: Antitrust in the New Gilded Age,” from which this essay is adapted.

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