American antitrust policy has long placed the interests of consumers above those of smaller rivals — often as measured by what a product costs — a restrained approach that has allowed companies to grow into trillion-dollar giants. But that consensus is shifting among economists and policymakers.

Still, Ms. Miller and her colleagues are threading a narrow political needle. The push for bold antitrust action faces resistance from many on the right, who argue that people like Ms. Miller exaggerate the problem they identify — and demand excessive solutions.

“The claim that across the board you’re seeing a diminution of competition in our economy because of lack of antitrust enforcement is overstated,” said Daniel Crane, a professor at the University of Michigan’s law school. He added that “breaking up existing companies is not a great way to start” or “terribly feasible from a legal perspective.”

The antitrust movement also has to build more support on the left.

K. Sabeel Rahman, the president of Demos, a progressive think tank, said the antitrust movement needed “a much better articulation of what the links are between concentrated corporate power and questions of racial equity and racial justice.”

Ms. Miller said she disagreed with the criticism from the right.

“If you look at meat processing, if you look at media, if you look at plagiarism detection software, if you look at baby formula, if you look at pacemakers, everywhere you look you see markets that have been rolled up and monopolized,” Ms. Miller said.

But she concurred with those on the left that making racial justice a part of the call for antitrust reforms was vital. She said her new group would engage in conversations “with groups that look at work through a civil rights lens” as it tapped into a populist anger that emerged from the financial crisis of 2008.

Ms. Miller spent 2010 to 2012 working for the Treasury Department as it managed the recovery. She said she had found herself “confused” and “dissatisfied” by what she saw as the department’s failure to help homeowners while shoring up big banks.