The Affordable Care Act envisions each state running its own exchange, but authorizes a federally-run fallback in states that don't act on their own.
States must decide by Friday whether they intend to move forward on their own. If not, they can either defer entirely to the federal government or handle the task jointly.
Republican governors in Nevada and Idaho have said they plan to operate their own exchanges, but most other GOP governors have held firm against state-run marketplaces.
Although their resistance ensures greater federal control over their states' healthcare systems, it's seen as an opportunity to strain the federal implementation effort.