A month after losing a landmark antitrust case brought by the Department of Justice, Google is headed back to court to face off for a second time against federal prosecutors.

In August, a judge ruled that Google has held a monopoly in internet search, marking the biggest antitrust ruling in the tech industry since the case against Microsoft more than 20 years ago. This time, Google is defending itself against claims that its advertising business has acted as a monopoly that’s led to higher ad prices for customers.

The trial begins in Alexandria, Virginia, on Monday and will likely last for at least several weeks. It represents the first tech antitrust trial from a case brought by the Biden administration. The department’s earlier lawsuit was first filed in October 2020, when Donald Trump was in the White House.

While U.S. officials have spent the past several years going after Big Tech, only Google has so far has ended up in federal court. The DOJ sued Apple in March, saying its iPhone ecosystem is a monopoly that drove its “astronomical valuation” at the expense of consumers, developers and rival phone makers.

In late 2020, the Federal Trade Commission filed an antitrust suit against Facebook (now Meta), claiming the company had built a monopoly through acquisitions of Instagram and WhatsApp. Earlier this year, Meta asked a court to dismiss the suit. In 2023, the FTC and 17 states sued Amazon for allegedly wielding its “monopoly power” to inflate prices, degrade quality for shoppers and unlawfully exclude rivals, undermining competition.

For Google, the focus turns to its ad tools, which are part of the company’s $200 billion digital ad business.

The government claims Google is in violation of Sections 1 and 2 of the Sherman Act, which prohibit anticompetitive behavior. The DOJ will argue that Google locked in publishers and advertisers to its products and that websites had to develop workarounds in response. A coalition of states, including California, Colorado, Connecticut, New Jersey, New York, Rhode Island and Tennessee, joined the case.

Google’s ad business has drawn numerous critics over the years because the platform operates on multiple sides of the market — buying, selling and an ad exchange — giving the company unique insights and potential leverage. In its initial lawsuit, the DOJ cited internal communication from a Google ad executive, who said owning multiple sides of the ad-selling process is like “if Goldman or Citibank owned the NYSE,” referring to the New York Stock Exchange.

At stake is how Google is allowed to operate its portfolio of ad products. The DOJ, if successful, seeks the divestiture of, at minimum, the Google Ad Manager suite (GAM), the marketplace that gives brands the ability to create and manage ad units and track ad campaigns and lets publishers sell ad inventory.

That’s different from Google’s flagship platform — Google Ads — which is primarily for businesses looking to advertise their products or services across search, websites, YouTube and other partner sites. 

In the most recent quarter, Google parent Alphabet reported ad revenue of $64.6 billion, accounting for over three-quarters of total sales. Of that amount, $48.5 billion came from search and other businesses like Gmail and Maps, and $8.7 billion came from YouTube.

The GAM suite is part of the Google Network business, which generated $7.4 billion in second-quarter revenue, or about 11% of total ad sales.

In addition to a potential partial breakup, Google could see a flood of litigation from advertisers seeking monetary rewards if the DOJ is successful. Bernstein analysts said Google could face up to $100 billion in such lawsuits.

In the first antitrust case, the court found that Google violated Section 2 of the Sherman Act, which outlaws monopolies. Judge Amit Mehta of the U.S. District Court for the District of Columbia agreed with the DOJ, which argued that Google has kept its share of the general search market by creating strong barriers to entry and a feedback loop that sustained its dominance.

“Google is a monopolist, and it has acted as one to maintain its monopoly,” Mehta wrote.

Google now awaits its punishment for that case. The DOJ is asking for an extended time frame, until February, to offer remedies, followed by a hearing in April. Google says the DOJ should have already done its homework and should be prepared to offer its proposal in October.

WATCH: Google’s antitrust woes mount