Byron York: Why do Democrats insist on going ahead with an overwhelmingly unpopular bill?

I've wondered why as well.

In the end, perhaps the most compelling explanation for Democratic behavior is that they are simply in too deep to do anything else. "Once you've gone this far, what is the cost of failure?" asks the strategist.
At that point -- Republicans will love this -- he compared congressional Democrats with robbers who have passed the point of no return in deciding to hold up a bank. Whatever they do, they're guilty of something. "They're in the bank, they've got their guns out. They can run outside with no money, or they can stick it out, go through the gunfight, and get away with the money."
That's it. Democrats are all in. They're going through with it. Even if it kills them.