Things could get really ugly.
We all knew this day would come. Things would be bad enough if Bush got to appoint a replacement for William Rehnquist, but at least Rehnquist was already known as a conservative justice. Anyone Bush would put up would retain the balance. O'Connor, while moderately conservative overall, was also a key "swing judge" on important issues such as abortion.
Now, I think the Democrats have gotten some of their spunk back when it comes to challenging Bush nominees, between the filibuster debate and John Bolton. I think that Bush knows this. Hopefully, he won't try to push anybody too radical through, but if he does, I have no doubt that it will be a contentious battle. And by "no doubt" I mean "for the sake of humanity, please." It's a given that we are going to get a conservative justice regardless of Democratic opposition. The question is how bad it will be.
And I have a feeling Ruth Bader Ginsberg will be the only woman on the court when this is over. There should be a requirement that the court alternates between five men and four women, and five women and four men, every time a position opens up. Realistically, I don't expect to see more than three women on the court at any time in my life, and I'm not betting on more than two.