“Given the story of Democratic Sen. Tim Johnson’s illness and surgery, it might be worth taking a few moments to understand how succession works. (This comes from Al’s Morning Meeting on Poynter. The full 12/14/06 blog posting can be found here.)
It is the 17th Amendment to the Constitution that sets out what happens when a senator cannot fulfill his or her duties in office.
The law sets down different rules for replacing senators and representatives. The sitting governor of the respective state has the sole power to select a senator’s successor. Traditionally, the governor chooses somebody from his or her own party. When a representative dies or retires or goes to jail, voters choose a successor in a special election.
All of that applies if the officeholder actually leaves office. But if a senator or a representative is alive, but incapacitated, no part of the law gives the governor the power to step in.
In fact, as the Associated Press points out, history reveals senators who could not even come to work and still kept their offices. In 1969, another South Dakota senator, Republican Karl Mundt, suffered a stroke. Mundt continued to serve until his term ended in January 1973, even though he was unable to attend sessions. He was even stripped of his committee assignments by the Senate Republican Conference in 1972.
That is not, however, the case with the president. For him, the 25th Amendment is the authority. If a president gets sick, for example, and can’t do his job, the VP can send a petition to Congress, allowing the representatives to decide whether or not the president is too sick to run the country.”
Then, I found this article in Slate, called “Capitol Hooky“, which is an excellent explainer of Congressional absences and the subsequent job effect. This has links to historical and statistical information.