Washington, D.C. ? Sen. Tim Johnson has been conscious at times since his emergency brain surgery last week, his spokeswoman said Monday. But he is being sedated so he can rest.

The South Dakota Democrat has made it through the first 72 hours since the Wednesday evening brain surgery, spokeswoman Julianne Fisher said, a benchmark that doctors consider a good sign for recovery.

The senator remains in critical but stable condition, she added.

Fisher said the next “target” for doctors is to watch his progress in the next week.

Johnson’s sudden illness has raised questions about the Democrats’ one-vote majority in the upcoming Senate session. South Dakota’s Republican governor, Mike Rounds, would appoint a replacement if Johnson’s seat were vacated by his death or resignation.

A Republican appointee would create a 50-50 tie and effectively allow the GOP to retain Senate control because of Vice President Dick Cheney’s tie-breaking vote.