Charles Krauthammer before the votes were counted:

If [Brown] wins health care is dead. They are going to have to start from scratch, if they want to, which would mean bringing in Republicans, rewriting it, tort reform, it would be completely different.
If Brown wins tomorrow the bill as we see it is dead. The only hope is if the House swallows the Senate bill whole, which I think is not going to happen. The only alternative is to delay the swearing in of the Republican in the Senate, and that would be catastrophic for the Democrats.
There is already a question of legitimacy hanging over the bill, a, because it would be the largest change in our social structure, a sixth of the economy, purely on a partisan vote, secondly, because you have huge resistance in public opinion, a spread of at least 10 points against it repeatedly and consistently.

Anthony Weiner (D-NY) is saying the same thing.