The Democrats know that once the committees have gotten together on a single bill, the Democrats absolutely must vote for it. If the bill fails, it sends a powerful signal that the Republicans can stop Obama and the Democrats whenever they want. That would effectively end Obama's presidency, it could cost the Congressional Democrats control of the Houses and possibly their jobs (1994 redux), and it could open the door to some loon like Palin winning in 2012. Any Democrat who risks destroying health reform, who risks screwing over his president and party leader, will be held accountable by the country and the party.
And specifically a guy like Baucus. He is already exposed on this: he averaged almost a million dollars a year in payoff money from the health sector, which is more than any Democrat in Congress; almost 30 of his staffers are now working as lobbyists. And he doesn't even need the money -- he won 73-27 last time. So he already looks bad. His committee is the only one dragging their feet on the reform issue; he looks like an idiot for trying to negotiate with Republicans who are clearly hell-bent on killing reform. His own constituents are already angry with his foot-dragging on the issue. He will get a big share of the blame if the effort crashes: his role in the disaster would be the first line in his obituary and the first line in his tiny entry in the history books, the incompetent boob who set the cause back 20 years. He could even lose his committee chair if Reid has the balls to demote him.
(Or on a more practical level, the Democrats could just proceed with the bills reported out by the four other committees, and ignore Baucus)
The Republicans know this issue is the key to hurting Obama and Democrats badly -- they've admitted it. For them, wrecking health reform is all about destroying the Democrats and regaining power. The fact that they are causing thousands of Americans to sicken, die, lose their homes -- they just don't care. That's why they've mobilized their goon squads, just as they did to steal the 2000 election. For them it is more important to attack America's Democrats, than to attack America's problems.
This is the message Obama will use to herd the Democrats: pass this bill, whatever it is, or we're doomed. The Republicans get that -- why don't you guys?