AMERICA is in the midst of one of the biggest dealmaking booms in its history. Since Lehman Brothers failed in September 2008, $11 trillion of mergers and acquisitions have been done or await completion. Activity this year has been at a record high. The latest marriage may be of Dow Chemical and DuPont, two of America’s oldest firms that together dominate the chemicals industry. They are talking about combining to create a $130 billion giant.

Antitrust regulators are finally losing their rag. Responsibility for policing deals is mainly shared between the Federal Trade Commission (FTC) and the Department of Justice (DOJ). So far in December the DOJ has scuppered the planned sale of General Electric’s electrical-appliance division to Electrolux, and the proposed sale of Bumble Bee Seafoods, owned by a private-equity firm, to a Thai rival. On December 7th the FTC said it would try to block the acquisition of Office Depot by Staples. A big railway takeover—Canadian Pacific wants to buy Norfolk Southern—is also causing a stink. Railroads have been at the heart of antitrust battles since the 19th century. Norfolk fears that the deal will not pass muster with the transport regulator.

Most merger waves eventually lead to blowback as ever more ambitious companies test the boundaries of what is permissible. In the late 1990s and early 2000s, regulators blocked deals in defence (Lockheed Martin and Northrop Grumman), telecoms (WorldCom and Sprint) and helicopter-making (Bell and Boeing) among others. European officials terminated General Electric’s takeover of Honeywell in 2001.

This time round, too, some big deals are attracting political heat. The public already detest some industries that have consolidated, or plan to, such as airlines and health insurers. On the campaign trail, Hillary Clinton has criticised plans to merge several large insurers. On December 8th Congress held hearings on the combination of AB InBev and SABMiller. “Nobody wants to take a seat at a bar and discover their only choices are between a Bud and a Miller,” grumbled Chris Coons, a thirsty senator. (The firms, if combined, promise to sell one of the brands of watery brew.)

There are more serious grounds for a tougher approach than politicians’ boozing preferences. In four-fifths of America’s big industries the top 50 firms had a higher market share in 2007 than in 1997. Profit margins are at a near-record high, hinting at a lack of competition. The scale of dealmaking—total transactions since Lehman’s failure amount to 46% of American firms’ current market value—points to more concentration, too. In the run-up to the financial crash many takeovers were by private-equity firms, which assembled vast portfolios of unrelated businesses. Now most deals are “strategic”, with two firms seeking to combine similar operations, boost their prices and cut costs. Food, cable TV, telecoms, airlines, computer chips and other industries all have consolidated in the past half-decade. Acquisitive firms are now trying clever tactics, which they hope will annoy regulators less. Dow and DuPont are likely to combine and then split again into three smaller firms, each with a speciality. Canadian Pacific has proposed putting itself into an independent trust, that will be combined with Norfolk but be run separately from it until regulators give the nod. If they do not, the two firms will be split again. But regulators are changing their approach, too. In the Bumble Bee case, the DOJ defined the market narrowly, examining the sale of just canned tuna, rather than all canned fish. And Nelson Fitts, a lawyer with Wachtell Lipton, a law firm, points out that in 2010 the DOJ and the FTC changed their merger guidelines. Rather than emphasising traditional yardsticks such as market shares in specific places, or barriers to entry, they are using a broader sense of whether a deal hurts competition—for example whether margins or prices will rise. In June a judge blocked the merger of Sysco and US Foods, arguing that they would have pricing power nationwide over restaurants, hospitals, hotels and other food buyers. A similar argument is being used in the case against Staples and Office depot: the FTC argues that it would raise prices for large businesses. How far will the antitrust backlash go? One measure is the number of pending deals where the shares of the target are trading at a big discount to the proposed offer price, suggesting that arbitragers and investors do not expect the transaction to go through. Of the 20 largest pending transactions nine are trading at a discount of over 10%, including the merger of Baker Hughes and Halliburton, two oil-services firms.

A few more nasty jabs from America’s regulators could create more uncertainty about the $1.4 trillion of deals outstanding that involve North American firms. It could also make executives thinking about new deals more nervous—one banker says that some chief executives are already getting cold feet about potential deals, when faced with a protracted period of wrangling with antitrust officials.

As antitrust regulators get tougher, firms that already have high market shares may come to fear being broken up, even if they won their position under their own steam, rather than through takeovers. Tech firms such as Google and Facebook may be deemed to dominate their industries. Microsoft barely escaped being dismembered in 2001. With America in a populist mood, antitrust is back in fashion and the tremors could even reach Silicon Valley.