Senator Johnson, a Democrat of South Dakota, has been conscious occasionally and is in “critical but stable” condition as he recovers from brain surgery, his spokeswoman, Julianne Fisher, said.

“He has been conscious at times,” Ms. Fisher said. “That’s when he is opening up his eyes and responding to voice commands and touch. But he is being sedated so he can rest and heal.”

Mr. Johnson, 59, underwent surgery at George Washington University Hospital in Washington on December 13 to stop bleeding in his brain and treat a malformation of arteries.

“He is still in intensive care,” Mr. Fisher said. “They will probably keep him there a little while longer.”

Control of the new U.S. Senate may depend on Mr. Johnson’s health. Democrats would have a 51–49 majority in the next session of Congress that begins January 4 if Mr. Johnson retains his seat. Were his seat to become vacant, Governor Rounds of South Dakota, a Republican, may appoint a Republican replacement, shifting the partisan balance to 50–50 and giving Republicans control through the tie-breaking vote of Vice President Cheney.

Last week, officials said Mr. Johnson has weakness on his right side and will need physical therapy.