Votes are not relevant
When a senator has to be away, he trades his absentee vote with someone who intends to vote the opposite way, who also needs to miss the vote. It's been done a long time and Republicans do it as often as Dems. Wouldn't surprise me if our glorius Senate Republicans made it harder or impossible for Kerry to do it now.
What is relevent is the behind-the-scenes work that the congressman's office performs. This might be suffering from his absence, but I have no information on it one way or the other.
Certainly you are right that it's impossible for the US to suffer from Bush concentrating on something other than the presidency.