to change on the provisions that could take down a site, it may be that many Democrats, who signed as co-sponsors are staying to try to "fix the bill". I know that was Senator Menendez's position.
I suspect that the reason the Republicans are scrambling is that their normal preference is for less regulation and they have to see how unpopular this bill is. It might also be identifying a split in the Republicans between those who see the issue as protecting property rights of major companies and those who see it just as regulating - negatively impacting their freedom. (ie think of it as a split between the Romneys of the party and the tea party.)
On the Democratic side, I think it is important to drop the idiotic claims that the Congressmen and Senators were "bought" on this. Protecting copyrights has been a long term issue and it is complex. I can remember any number of Democrats who have spoken of the need to get China to honor US intellectual property rights. There is no reason to always assume the worst and attack the integrity of anyone supporting something you do not like. ie Al Franken is attacked on this thread. Could it be that because of his former career, he is sensitive to the fact that many entertainers end up cheated out of the full amount they should get from their work because many get it for free?
My guess is that the Democrats' attempt to fix the bill will not produce a bill that can pass both houses. I would not be surprised to see more Republicans desert SOPA -which was their bill - and PIPA. Even if it really is fixed, I would assume that many will not see that and the people who support it will suffer some political damage. It very likely might be better to abandon it and some time in the future restart the entire process - and this time include more internet savvy people in trying to define the solution.