BIG TECH, meet Big Brother.

The Justice Department announced Tuesday a broad-based antitrust review of “the practices of market-leading online platforms,” without naming any names — and without explaining exactly what practices officials believe could be anti-competitive. The country is still catching up to a new era in which a handful of Internet companies have substantial power over Americans’ daily lives, and that power is worth examining. But the scope and circumstances of the investigation announced this week are cause for caution.

Increased interest across the government in scrutinizing top tech firms’ market behavior signals the erosion of the argument that platforms providing free services cannot possibly run afoul of authorities. Some critics believe industry giants are unfairly leveraging their dominance to bury rivals on the Web where Americans cannot see them or sucking up their data to drive them out of the game. Others think so much control resting with so few is itself a problem. President Trump and his allies, however, have seized on a highly specific grievance: Silicon Valley is too liberal, and its leaders are censoring conservatives online.

There is no evidence to support claims of censorship, yet conservatives have turned multiple oversight hearings into circuses full of rhetorical acrobatics and accusations. Only this month, Mr. Trump convened a “social media summit” similarly designed to bully companies for not behaving the way he wants. Last week, he promised to “take a look” at Google after allegations that the company was committing treason by working with the Chinese government. And in June, he explicitly connected Facebook, Google and Amazon to his electoral prospects. (Amazon founder and chief executive Jeff Bezos owns The Post.) “The real collusion is between the Democrats and these companies,” he told CNBC. “They were so against me during my election run. Everybody said, ‘If you don’t have them, you can’t win.’ Well, I won. And I’ll win again.”

It is wishful to think that an increasingly political Justice Department operates independent of these pressures. We already know the president reportedly sought to push authorities to block a major media merger because of his animosity toward CNN. And last September, officials said outright that a meeting with state attorneys general about antitrust would revolve around concerns that companies were “intentionally stifling the free exchange of ideas on their platforms” — giving the lie to the argument that enthusiasm for a probe is untethered from the president’s perennial personal gripes.

Antitrust investigations should be targeted to particular practices by particular companies that hurt consumers or impede competition. They should not be a cudgel brandished against an entire industry for a vague set of violations that could well include having Democratic employees and leaders. Antitrust advocates worried about the concentration of power in one place should take care not to champion an even more dangerous accumulation in another.

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