The public option had been kicking around for a while, however, in policy-wonk circles. Giving the uninsured an opportunity to purchase coverage through a Medicare-like health plan was seen as a useful means of putting competitive pressure on private insurers to provide decent coverage at low prices.
But as the debate has progressed, the public option has become an ideological flash point, igniting fears on the right that it will be the precursor to a government-run system like Canada's and some European countries'. Which is the same reason that many on the left like the public option so much.
What is getting lost in all the shouting, though, is that its effects would be limited. The vast majority of Americans--those who get their coverage through larger employers--would not even be eligible to participate at first. And it wouldn't be free: those who enrolled would have to pay premiums, just as they would to a private insurance company.