It's not clear if he's including Lieberman as a Democrat (I'm assuming he is including Sanders), but here's what Senator Harkin, Chairman of the Senate HELP committee, had to say:
Pointing to a Thursday meeting of Senate Democrats in which most members of the caucus expressed support for a government-run insurance plan, Harkin said only five members of his party are holding up progress toward a such a plan.
If someone else diaried this, I'm sorry. This seems like a big deal and the most specific count yet I've seen on this question. Of course, the question remains: what kind of public option. I'm assuming (heaven help me Felix Unger) that Harkin means the public option of the kind he (and we) support, namely a robust one. We'll see.