The Sherman Antitrust Act was passed in the Senate by a 51-to-1 vote, while the House endorsed it unanimously. It was two pages long, did not bother defining the word “monopoly” and was used in its first decade mostly to go after labor. Some historians think the conservative Congress that passed it was merely being cynical, offering a sop to reformers.

The fear of the future, however, was real as the economy transformed itself. In 1890, the percentage of the population in agriculture was at 43 percent, down from 72 percent in 1820. The steel companies, meatpackers and railroads hired and then hired some more. The number of millionaires rose from a few hundred before the Civil War to about 4,000 in 1890, historians estimate. .

“The rising fear of the day was that honest, hardworking people are being screwed and the system is rigged,” said Mr. O’Donnell, author of “Henry George and the Crisis of Inequality: Progress and Poverty in the Gilded Age.” People grew to depend on big companies like Standard Oil, just as their descendants today cannot sign off from Facebook and Google. But they did not necessarily like them.

“Nearly every household had a five-gallon red can of Standard Oil’s kerosene,” Mr. O’Donnell said. “But the founder, John D. Rockefeller, was often seen as rapacious octopus.”

In 1900, the Democratic Party platform included the belief that “private monopolies are indefensible and intolerable,” declaring: “Unless their insatiate greed is checked, all wealth will be aggregated in a few hands and the Republic destroyed.”

Groundbreaking reforms followed, including the breakup of Standard Oil. In 1913, in a largely forgotten episode, National Cash Register Co. became the first tech company to be successfully prosecuted for violating the Sherman Act. Nearly 30 executives at the company, which controlled 95 percent of the cash register market, were convicted and sentenced to brief prison terms. While they appealed and never served a day, the company’s power was broken.

Antitrust moves ebbed and flowed through the 20th century. In the late 1940s, said Daniel Crane, an antitrust historian at the University of Michigan, “there was quite a bit of alarm about the growing tide of industrial concentration following the war, and fear that undue levels of concentration could bring fascism or communism to power.”