There’s pretty strong precedent for injured/ill members of Congress retaining their seats.
In addition to Tim Johnson a few years ago, I can recall as far back as Charles Sumner:
http://en.wikipedia.org/wiki/Charles_Sumner#Antebellum_career_and_attack_by_Preston_Brooks
Who stayed away from the Senate for three years after he was severely injured. Others that come to the top of my head would include Joe Biden in 1988.
It would be really bad form - and bad PR - to attempt anything of the sort.
And if a dead Democrat gets elected, the spouse serves.