There’s another reason none of the Democrats who oppose the Senate bill from the left are likely to vote no, besides the very good one Phil mentioned: the vote is too close and no liberal representing a strong Democratic district can afford to risk being the deciding vote against health care reform. It would potentially be a career-ender.
Dennis Kucinich is a case in point: he attracted viable primary opposition last time around and a decisive vote against health care could make the difference in whether he turns such challenges back in the future. The only way liberal defections would be permitted is if the bill was clearly going to pass, which obviously isn’t the case right now, or if it was clearly going to be defeated, which isn’t the case either. (It’s also unlikely that Nancy Pelosi would bring the bill to the floor in that event.)
Prediction: No generally liberal Democrat will vote against the health care bill on anything other than pro-life grounds. Speaking of pro-life liberals, that’s another issue on which Kucinich flip-flopped in spectacular fashion.