How are Hillary Clinton and Barrack Obama able to maintain their responsibilities of senators of a state (whatever they even are) while running a full fledged campaign for presidency of the United States Of America? If I were a resident of New York or Illinois I’d feel that my best interests were not being looked after.
As a resident of Illinois, I’m not too worried. Incidentally, you forgot Arizona’s senator in your question.
Most Senate votes are foregone conclusions and the absence of the candidates doesn’t matter much. If it’s a real fight to pass a bill, the candidates will fly back to DC for the vote. They’ve already done so for a few bits of partisan legislation where every vote counted.
They do miss out on some committee meetings and stuff but much of that info can be sent to them. for that matter, most of what senators “do” is handled by their staff anyway. In this modern age, it’s very easy to telecommute and get work done from Indiana or Pennsylvania or North Carolina.
Besides, it’s about time that slacker Durbin started carrying his weight 
Plus, up until last summer, I was an Illinois resident, and I liked the fact that a Senator from my state could possibly be the next president. So any time he spent away from his FT job was pretty much excused in my eyes.
And like Jophiel said, li’l Dickie Durbin can always pick up the slack. (Or in New York’s case, li’l Chuckie Schumer.)
Their work certainly suffers. Yes, the staff can do a lot but without a Senator the staff is limited (I speak as a former Senate staffer). If the Senator isn’t there to offer amendments during a committee mark-up, for instance, the staff is pretty impotent to do anything. If he/she isn’t there to go to meetings with other Senators or Administration officials to advace the agenda, the staff can’t do much. If he/she isn’t there to place a phone call to a committee chairman to ask for a favor, the staff can’t do much.