For on-the-ground organizations, Obamacare represents a once-in-a-generation organizing opportunity. By signing someone up for health insurance, they are delivering a tangible benefit, something that person will value for years to come, and winning loyalty along the way. Nonprofits, as well as mayors and governors, have an intense incentive to make Obamacare work.

And millions of those uninsured people — many of them young and healthy — are in tightly concentrated urban areas, target-rich environments for the grassroots groups and politicians working to sign people up.

Take Los Angeles County. More than 2.2 million uninsured people live there — nearly 5 percent of the entire country’s uninsured population. In the Houston metro area, there’s Harris County with more than 1.1 million uninsured. Add in Cook County, which includes Chicago, and Miami-Dade County in Florida, and the numbers of concentrated and targetable uninsured quickly add up.

The importance of these counties to Obamacare’s success is not lost on anyone. “California is ground zero for this,” said Steven Abramson, a marketing manager for the Community Health Alliance of Pasadena. “And LA County is ground zero for California.”