As Jon Cohn rightly points out, John Edwards probably has the best of the Man Date positions, favoring a (slightly vague) version of automatic enrollment: Every time you come into contact with the medical system, you'll be signed up for a plan. "Basically," Edwards says, "every time [people] come into contact with either the healthcare system or the government, whether it's payment of taxes, school, going to the library, whatever it is they will be signed up."
I'm a bit curious as to how that'll work -- how does it interact with the subsidies? What happens if someone just says no? How do you force them into the billing process? Is the public plan the default? -- but as a basic approach, it's probably quite achievable, and is good way to basically erase the uninsured population. To think of it in a sightly clearer way, his individual mandate works a bit more like a government mandate -- the onus is less on the individual to seek coverage, and more on the government to present them with it. Assuming the policy works, the individual mandate would really only exist in odd cases when individuals simply refused to accept insurance -- in which case, it would be like if they declined to pay their taxes. Beyond that, it would largely be the government sending you a form that either enrolls you in insurance, or records your preexisting enrollment.
So, as it stands now, Edwards has the best, and most defined, mandate plan. Hillary is second. And Obama, for all his talk, simply doesn't have one. Some say, "well, these mandate plans don't actually establish universality. I know this guy Bob, right, and Bob lives in Montana, and he's got, like, sixty fucking guns, big guns, and also a butterfly knife, and he hates the government, and won't sign up!" Maybe so. But these plans actually strive towards universality and construct mechanisms which could plausibly achieve something very, very close. That Obama's doesn't is a real failing, both on the policy merits and on the politics. If he thought man dates were too unpopular, or too ineffective, he should've been bolder in response, not more timid. That's not what his campaign was sold as.