"Too many poor people are showing up to hospitals without health insurance and getting treated for free, driving up the cost for everyone else. We need to FORCE them to buy health insurance or else there is a penalty!"
Doesn't sound that great in those terms, but that's exactly what you got. Change the way you talk about it, sprinkle in a couple keywords like "affordable", mix in some coverage for pre-existing conditions, and suddenly its more palatable to liberals.