As has been covered extensively in the news, South Dakota Senator Tim Johnson has very recently undergone surgery to deal with a brain hemorrhage. Were Johnson to resign his post before the new session convenes, functional control of the Senate would likely return to the GOP (the Republican governor of South Dakota would likely nominate a Republican as a replacement, leading to a 50/50 split, with VP Cheney as a tiebreaker).
While some conservative commentators hover ghoulishly over the stricken Senator Johnson, a guest on NPR's NewsHour program reminds us of California Senator Clair Engle, who did not resign his position as his brain cancer progressed:
On June 10, 1964, during the roll call for the historic, successful effort to break the filibuster on what would become the Civil Rights Act of 1964, when the clerk reached "Mr. Engle," there was no reply. A brain tumor had robbed Senator Engle of his ability to speak. Slowly lifting a crippled arm, he pointed to his eye, thereby signaling his affirmative vote ("aye"). Few who witnessed Engle's brave act forgot it. Nine days later the Senate approved the act itself—producing one of America's towering 20th century legislative achievements.Senator Engle died a month and a half later.