WASHINGTON — Obama administration officials are getting ready to set up and operate new health insurance markets in about half the states, where local officials appear unwilling or unable to do so.

The markets, known as exchanges, are a centerpiece of President Obama’s health care law, and running them will be a herculean task that federal officials never expected to perform.

When Congress passed legislation to expand coverage two years ago, Mr. Obama and lawmakers assumed that every state would set up its own exchange, a place where people could shop for insurance and get subsidies to help defray the cost.

But with Republicans in many states resisting the creation of exchanges or deterred by the complexity of the task, federal officials are preparing to do the job, with or without assistance from state officials.