US Senator Tim Johnson of South Dakota remains critically ill but stable after brain surgery for a potentially deadly bleeding that could deprive Democrats of their thin Senate majority.
Doctors said Mr Johnson had needed no additional surgery and was having an uncomplicated recovery. A Johnson spokesman warned that could change and the next 24 to 48 hours were as "a critical benchmark."
Tim Johnson
Democrats narrowly wrested control of the US Congress from Republicans in last month's elections, gaining a thin 51-49 majority in the Senate when it convenes in three weeks.
If Mr Johnson (59) were to leave office, Republicans could gain control of the Senate, with Vice President Dick Cheney having the power to break tie votes.
"We're all praying for a full recovery, we're confident that will be the case," said Senate Democratic leader Harry Reid of Nevada.