Sebelius plans aggressive ObamaCare push in Fla.

Florida also refused to establish its own insurance exchange, punting to the federal government to set up the new marketplace. The state led the anti-ObamaCare lawsuit that ended up at the Supreme Court last year.

And though Gov. Rick Scott (R) endorsed the law's Medicaid expansion, the GOP-controlled state legislature refused to sign on.

"Since the state is doing everything in its power to make it more difficult for families to access coverage, we are doubling down on our efforts to reach Floridians to ensure they  know the truth about the benefits that they are entitled to and how to access the coverage they need," Peters said.

Sebelius will spend the "next three business days" in Florida, at events alongside administration allies promoting enrollment in the law's new coverage options.

Florida is key to the administration's enrollment push. Roughly one-third of the uninsured young adults the administration is targeting live in just three states — California, Texas and Florida. And because Texas and Florida have both rejected the law, HHS has its outreach work cut out for it.