Tim Johnson (D-SD) is in critical condition following brain surgery. He has a unique congenital condition that causes enlargement of blood vessels to the brain, which results in entangling the vessels and limiting blood flow.
If he cannot serve, South Dakota's Republican governor would appoint his replacement -- in a 51-49 Senate, that would mean a 50-50 split with VP Cheney casting deciding votes in case of a tie.
Johnson won a highly disputed election (to say the least) in 2002. His opponent, John Thune, refrained from challenging the election and instead ran again -- this time against obstructionist-in-chief Tom Daschle. Thune won.
Johnson turns 60 in a few days. Politics are irrelevant to his health: get well soon.