From the Washington Post:
The Senate appears deadlocked over legislation that would require members to file their campaign finance forms electronically — the method used by their House counterparts, presidential candidates and the majority of state lawmakers.
Instead of submitting forms electronically, senators print their reports and deliver them to the clerk’s office. The staff there scans them into a computer and transmits them electronically to the Federal Election Commission. The FEC then prints the forms again and hires workers to type the information into a database so they can be made public online.
The practical result is that it can take months for Senate campaign filings to become publicly available. Names of donors to a particular campaign often will not be known until long after Election Day has come and gone.