WASHINGTON (CNN) -- The Democrats' slight hold on power in the Senate is largely safe despite South Dakota Democratic Sen. Tim Johnson's health scare, Senate Historian Don Richie tells CNN. As stipulated by Senate rules, Johnson could retain his seat even if he is incapacitated, unable to vote, and not even able to show up to work.

Moreover, the Senate does not have the power to forcefully remove Johnson unless he committed a crime.

Such a scenario has even occurred in Johnson's home state of South Dakota. After South Dakota Sen. Karl Earle Mundt had a stroke in 1969 he remained in office until his term expired in 1973 without casting another vote after the governor refused Mundt's wish of appointing his wife to the post.

A state governor has the power to appoint a new senator only if the current senator dies in office or resigns his seat.