Democratic control of the US Senate has been cast into doubt after a senator underwent brain surgery.
Senator Johnson and his wife have both had cancer
Senator Tim Johnson, 59, of South Dakota, suffered from bleeding on the brain, a US Capitol doctor said, describing the surgery as successful.
The Democrats captured control of the upper house of Congress by a single seat in elections last month.
If Mr Johnson stands down, the Republican governor of South Dakota will name his successor.
That person - likely to be a Republican - would serve until the next general election in 2008.
Mr Johnson was admitted to George Washington University hospital at midday on Wednesday after experiencing what his office initially said was a possible stroke.
US Capitol doctor John Eisold issued a statement saying that Mr Johnson suffered "an intracerebral bleed caused by a congenital arteriovenous malformation".
In a later statement he said the senator "has continued to have an uncomplicated post-operative course".
Dr Eisold said Mr Johnson was responding "appropriately" to "both word and touch" and "no further surgical intervention has been required".
Mr Johnson's wife, Barbara, said his family was "encouraged and optimistic".
President Bush wished Mr Johnson a speedy recovery through a spokeswoman who said the president's thoughts were with Mr Johnson's family.
Mr Johnson, who turns 60 at the end of December, had prostate cancer in 2004 but says he is now clear of the disease following an operation.
His wife has also had cancer.
There is little precedent for forcing a living senator to stand down against his will.
A predecessor of Mr Johnson, Karl Mundt, continued to hold his Senate seat for three years after a stroke that incapacitated him in 1969, although he was unable to attend Senate sessions.