While presidential and House candidates file their campaign finance reports electronically, Senate candidates file theirs on paper. It takes the Federal Elections Commission months to process these paper records and put them online, where the public can see them. The upshot: voters don't know who donated to Senate candidates in the last three months of their campaigns. Did Republicans fund Joe Lieberman's last minute push, for example? You could only find out after you've voted.
The good folks at the Sunlight Foundation have identified which Senators are working for more transparency (primarily Russ Feingold) and which ones are working for less (primarily John Ensign). And they also know how you can take action. Check out their web video.