WASHINGTON — The Obama administration is about to carry out a major provision of the new health care law by issuing standards for health insurance exchanges, the markets where consumers and small businesses will be able to buy coverage from competing private plans.

To encourage states to set up the exchanges, federal officials said, they will give state officials broad discretion to decide the operational details. However, the federal officials made clear that they would set up and operate an exchange in any state that refused to do so.

Federal officials said the rules showed how President Obama was moving to expand insurance coverage, even as critics attacked the health care law in Congress, in court and in campaigns for the White House and Congress.

The rules, to be issued within days, were described by two officials: Kathleen Sebelius, the secretary of health and human services, who testified at a Senate hearing on Wednesday, and Timothy B. Hill, a senior official at the department, who provided additional details on Thursday at a conference of America’s Health Insurance Plans, a trade group.