Those without health insurance have less than two months to enroll in new plans before penalties kick in, and the Obama administration is racing the clock to get them signed up.

A new study conducted for The Associated Press shows the administration is best off focusing on a relatively narrow geographic area: Half of those under 65 without insurance live in just 116 of the nation’s 3,143 counties. And half of all 19-39 year olds without insurance — the most coveted demographic as health-care providers look to expand their risk pools — live in 108 counties.

Thirteen heavily urban counties are home to 20 percent of the country’s uninsured, according to the study, conducted by the State Health Access Data Assistance Center at the University of Minnesota.

More than 2 million people without insurance — 5 percent of the national total — live in Los Angeles County, the biggest pool of potential sign-ups, while more than a million uninsured live in Harris County, Tex., home of Houston. Nearly 30 percent of all Harris County residents are without insurance. Hundreds of thousands without insurance live in Cook County, Ill., Miami-Dade County, Fla., and Dallas County, Tex., the third, fourth and fifth-largest pools of uninsured.

Federal officials are focusing on 25 metro areas, the AP said, including Dallas, Houston, Miami, Atlanta, northern New Jersey, Philadelphia, Detroit, Cleveland and Indianapolis. The feds are less concerned with cities like Los Angeles and New York, where states are running their own health-care exchanges.

The University of Minnesota Center’s data shows the lowest number of uninsured Americans live in the Midwest and the Northeast, where many states have expanded Medicaid to cover additional low-income residents under the Affordable Care Act. But even before those expansions, the American Community Survey showed low rates of those without insurance in the Dakotas, Minnesota, Iowa, Wisconsin, Illinois, New York, Connecticut, Massachusetts and Vermont.

About 14.9 percent of Americans lack health insurance, according to the center’s estimates. The percentage of Americans covered by public health insurance plans jumped in 28 states between 2011 and 2012.