Democrats and Republicans in Washington make common cause on few issues these days, but recently they have found a common enemy: Google.

On Tuesday, the Justice Department filed an antitrust lawsuitaccusing the tech behemoth of illegally protecting its monopoly over online search services and the ads that run on them, a move that could prove to be the government’s most significant challenge to a tech company’s market power since it took on Microsoft in 1998.

Republican attorneys general from 11 states have signed on to the lawsuit, but it has also earned support from high-profile Democrats like Letitia James, the attorney general of New York, and Senator Elizabeth Warren.

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 As Matt Stoller wrote in The Times last year, many news organizations have virtually no choice now but to depend on Google and Facebook to reach readers and fund their operations. This centralization of power, Mr. Stoller argues, poses two crises: a financial crisis, because advertising revenue that used to go to publishers is now captured by big tech intermediaries, causing the news industry to collapse; and an editorial one, because media’s dependency on a handful of platforms incentivizes virality and sensationalism over high-quality journalism.