I think I've mentioned this before, but Paul Krugman flagged a "comment of the moment by Alice Olson which reads

“ I, for one, am glad the Voting Rights Act, Medicare and Medicaid didn't have opt-out options.”
— Alice Olson
Opt-out public option

Olson and Krugman seem not to know that Medicaid is an opt-in program.

Opt-in is much worse than opt-out. For one thing, with opt-in the country ends up with 50 odd different programs -- as in the case of Medicaid. Prof Krugman, recall C. Romer's speach on how the stimulus is working. One of the best pieces of evidence used differences in Medicaid programs across states as an instrument for stimulus spending across states.

Use the google or Wikipedia it

State participation in Medicaid is voluntary; however, all states have participated since 1982 when Arizona formed its Arizona Health Care Cost Containment System (AHCCCS) program.