The biggest U.S. challenge so far to the vast power of today’s tech giants has reached its climax.

On Thursday and Friday, lawyers for the Justice Department, state attorneys general and Google will deliver their final arguments in a yearslong case — U.S. et al. v. Google — over whether the tech giant broke federal antitrust laws to maintain its online search dominance.

The government claims that Google competed unfairly when it paid Apple and other companies billions of dollars to automatically handle searches on smartphones and web browsers. Google insists that consumers use its search engine because it is the best product.

In the coming weeks or months, the judge who has overseen the trial in U.S. District Court for the District of Columbia, Amit P. Mehta, will deliver a ruling that could change the way Google does business or even break up the company — or absolve the tech giant completely. Many antitrust experts expect he will land somewhere in the middle, ruling only some of Google’s tactics out of bounds.

The trial is the biggest challenge to date to the vast power of today’s tech giants, which have defined an era when billions of people around the world depend on their products for information, social interaction and commerce. American regulators have also sued Apple, Amazon and Meta in recent years for monopolistic behavior, and Google’s case is likely to set a legal precedent for the group.

“This will be the most important decision and the most important antitrust trial of the 21st century,” said Rebecca Haw Allensworth, a professor at Vanderbilt Law School who studies antitrust. “It’s the first of the major monopolization cases against the major tech platforms to go to trial, and so that makes it a bellwether.”

The Justice Department declined to comment. A spokesman for Google pointed to an earlier statement from one of the company’s executives that the evidence from the trial confirmed that people “have many choices when searching for information online, and they use Google because it’s helpful.”

At the heart of the case is Google’s dominance in online search, which generates billions in profits annually. The Justice Department says Google’s search engine conducts nearly 90 percent of web searches.

The company spent $26.3 billion in 2021 alone to become the default search engine on browsers like Apple’s Safari and Mozilla’s Firefox, meaning it is automatically selected for users out of the box, according to information presented at the trial. Apple’s share was about $18 billion, The New York Times has reported.

When the Justice Department sued Google in 2020, it argued that those contracts were designed to defend its search business monopoly and hurt the ability of other companies, like Microsoft and DuckDuckGo, to compete.

Months after the federal lawsuit was filed, a group of state attorneys general filed their own antitrust case against Google over its search business and made similar allegations. Judge Mehta heard the cases together over 10 weeks last fall.

Lawyers questioned experts and executives, including Google’s chief executive, Sundar Pichai, and Microsoft’s chief executive, Satya Nadella.

Mr. Nadella said that the rival’s dominance made the internet the “Google web,” and that he feared a future in which Google would use similar tactics to dominate the booming field of artificial intelligence.

“Despite my enthusiasm that there is a new angle with A.I., I worry a lot that this vicious cycle that I’m trapped in could get even more vicious,” Mr. Nadella testified.

Google’s Mr. Pichai later testified that the company had created a better experience for consumers on the web through products, like the Chrome web browser, that used Google as its search engine.

Once closing arguments conclude, Judge Mehta must determine whether Google has monopoly power over the two products at issue in the case: general search engines and the ads that run in search results. To do so, he may look at Google’s overall share of the market and whether its power over search can be disrupted by competitors.

Then, if he determines that Google has monopoly power, Judge Mehta will decide if the company broke the law by making agreements to be the default search engine on smartphones and web browsers to defend its market share.

Legal experts say he could issue a mixed ruling, one likely to involve scrapping some of the government’s allegations but ruling that some of the contracts and policies highlighted during the trial do amount to legal violations.

If the judge rules against Google in any way, he will also ultimately have to determine how to correct the illegal behavior, for example potentially instructing the company to terminate its default search engine agreements with Apple and others. During this phase of the trial, Google and the government could both have the opportunity to present Judge Mehta with their arguments over how best to address any issues identified in the case.

The judge could also look to the European Union, where in 2019 Google offered smartphone users the ability to choose their default search engine in an attempt to comply with an earlier antitrust ruling by regulators against the company. Though that in theory gives smaller search companies more of a chance to compete with Google, many rivals complain it doesn’t work.

While the government has not yet revealed what it may ask for if the judge rules in its favor, it could ask Judge Mehta to make structural changes to Google’s business, for example breaking off a division that helps the company capture search queries, like its Chrome browser. That would be a more surprising option, experts said.

“I think it’s unlikely that D.O.J. will seek some sort of breakup here,” said Bill Baer, a former head of the Justice Department’s antitrust division. “It’s more likely there will be some sort of restrictions on Google’s behavior going forward.”

In the late 1990s, the Justice Department mounted a lengthy antitrust challenge to Microsoft’s dominance, saying it had used its power over operating systems to lock out some of the earliest web browsers. The company ultimately settled with the government, agreeing to give computer makers more options to feature software that wasn’t made by Microsoft. The legal standards set by that case were referred multiple times throughout the Google trial.

Now, Judge Mehta may similarly shape the tech industry and broader regulatory efforts to rein in its tactics.

Mr. Baer said the ruling could help draw the lines for what behavior is fair game for a company that operates a valuable and thriving platform and wants to defend its dominance. That would apply not just to Google’s search engine. Apple’s App Store, Amazon’s marketplace and Meta’s many social networks are all platforms — and face many of the same antitrust questions.

“The facts are somewhat different,” Mr. Baer said. “But there’s a lot in common in terms of: At what point does your behavior intended to exclude rivals constitute an unlawful effort to maintain your monopoly position?”