Google’s chief executive, Sundar Pichai, told a federal judge on Wednesday that a government proposal to break up the company would hobble the business, as he aimed to stave off drastic changes to fix an illegal monopoly in online search.

Judge Amit P. Mehta of the U.S. District Court for the District of Columbia ruled last year that Google had broken the law to maintain a search monopoly. This month, he convened a hearing to decide on the measures, known as remedies, that would be put in place to address the illegal behavior.

As the company’s second witness, Mr. Pichai was called to make the case that the court should avoid the government’s aggressive solutions, including forcing Google to sell its popular Chrome web browser and share data with rivals. Mr. Pichai said that the government’s proposal would lead the company to make fewer investments in new technology if it needs to share the benefits with its competitors for a minimal fee.

“The combination of all the remedies, I think, makes it unviable to invest in the R&D the way we have for the past three decades, to continue to innovate and build Google search,” he said, referring to research and development.

Mr. Pichai is the highest profile witness expected to testify at the landmark three-week hearing, which could rebalance the power dynamic in Silicon Valley. The tech industry is locked in a race to develop internet products powered by artificial intelligence, and new restrictions on Google’s business could supercharge its rivals’ efforts and hamper its own.

The Google search case is also the first major test of American government efforts to restrain tech giants’ enormous power over commerce, communications and information online. A federal judge in Virginia ruled this month that Google was also a monopolist in some online advertising technology.