Ask HN: Technology/tool for tracking Congressional bills?

I've been thinking about this for a while, but I'm not sure if it already exists in some form or another. Is there a tool that Congress (and state and local governments) use for tracking line by line changes publicly of proposed bills at their various revisions? Technology-wise it seems to simply be a group wiki and a very simple problem to solve.

In a 1,000 page bill then you could see exactly what senator inserted which lines (or give group responsibility/credit to a section), and who pushed for what changes. No longer would it simply be about blaming people for who voted for what, but we could have transparency into the process of the actual writing/revision of the bills. Then we would know for sure who was responsible for various lines and items (earmarks) and those could be easily tracked.

Does this exist? Is it in use by the US Congress or any state senates? I can't imagine what citizen would be against this type of transparency of government.