States will be able to set their own standards for determining which insurers can sell medical coverage in online marketplaces that will serve an estimated 24 million people under the U.S. health-care overhaul.

Rules announced today by the Obama administration allow states to determine whether insurers led by UnitedHealth Group Inc. must bid to compete in the exchanges, or meet general standards set by the state that fit within broad federal guidelines. They also allow states to pool their exchanges.

The health law signed in 2010 directed states to set up websites and telephone lines that enable residents to compare and buy coverage as if shopping for plane tickets. The rules come two weeks before the U.S. Supreme Court hears arguments on whether a provision mandating coverage for all Americans is constitutional, draining some of their impact, according to Thomas Carroll, an analyst with Stifel Nicolaus & Co.