Johnson in critical condition after brain surgery

South Dakota Sen. Tim Johnson is in critical condition this morning after undergoing emergency brain surgery over night. A source close to Johnson tells CNN that the surgery was "successful" and said that anyone who undergoes such a procedure is deemed to be in critical condition, at least initially. "The next 24 to 48 hours are critical," the source says.

It's still not clear what happened to Johnson Wednesday. He was on a conference call with reporters a little after noon when he began to stutter. He seemed to recover well enough to finish the call, but his condition deteriorated rapidly after that. His office initially said he was being treated at George Washington University Hospital for a stroke or "stroke-like symptoms," but Johnson's spokeswoman later said that tests had shown that he had suffered neither a stroke nor a heart attack.

In the event that Johnson were to die, South Dakota's governor would be entitled to appoint someone else to serve the remainder of his term, which ends in 2008. Assuming that the Republican governor would appoint a Republican senator, the Democrats' effective 51-49 advantage in the Senate would become a 50-50 split, with Vice President Dick Cheney providing the tie-breaker for the GOP.

That's not an issue if Johnson recovers, of course. It also may not be an issue if he survives but remains incapacitated. There's nothing in the Constitution, the law or Senate rules that requires a senator to serve only in good health. And indeed, as USAToday reports, there is precedent for a senator to continue serving despite serious health problems. Democratic Sen. Clair Engle of California, unable to speak after having suffered a brain tumor, cast a critical "aye" vote to end a filibuster on the 1964 Civil Rights Act by pointing to his eye. And Republican South Carolina Sen. Strom Thurmond served the final year of his last term in the Senate while living in a hospital room at Walter Reed.

-Tim Grieve