It’s an unfortunate event for the Senator and it could have profound implications:
Democratic Sen. Tim Johnson of South Dakota suffered a possible stroke Wednesday and was taken to a Washington hospital, his office said.
Johnson became disoriented during a call with reporters at midday, stuttering in response to a question. He appeared to recover, asking if there were any additional questions before ending the call.
If he should be unable to continue to serve, it could halt the scheduled Democratic takeover of the Senate. Democrats won a 51-49 majority in the November election. South Dakota’s governor, who would appoint any temporary replacement, is a Republican.
I sincerely wish him a speedy and full recovery. His Wikipedia bio:
Johnson is one of the more conservative Democrats in the Senate.  While in the House, he was among the minority of his party to vote in favor of the Personal Responsibility Act of 1995, a welfare reform bill, and another bill to repeal the ban on semiautomatic weapons. He was among the minority of Democrats to vote for Bush’s 2001 tax cut. On January 31, 2006, Johnson was one of only four Democrats to vote to confirm Judge Samuel Alito to the U.S. Supreme Court. He has also called for “broadened use” of the death penalty. 
Johnson was, however, among the minority of senators to vote against the Laci and Conner’s Law which was strongly supported by pro-life groups. While a member of the House, he was one of only 16 congressmen to vote against the Telecom Act of 1996, which provided for deregulation and competition in the communication sector and was given firm support by Republicans, business groups, and most Democrats.
His elder son, Brooks, serves in the U.S. Army, making him one of the few federal elected officials with a child in the U.S. Armed Forces. Michael Moore states in his film Fahrenheit 9/11 that only one member of Congress has a son serving in the military; Moore was referring to Johnson, although he didn’t mention his name. He and his wife Barbara, a professional social worker, have another son, Brendan, and a daughter, Kelsey.
Blogger News Network: I hope that Senator Johnson (who is very hard to dislike even though he is a Democrat) has a full and speedy recovery.
Chuckoblog: It disgusts me that Big Media have taken this opportunity to make this into a control of Congress issue. I wish Tim Johnson the absolute best. Though I may disagree with his choice of party, and probably with his ideology, he and his family deserve our prayers.
Hot Air: His condition is unknown. Please keep his health and safety in your prayers.
Suitably Flip: While concerns for Senator Johnson, his family, and his speedy recovery are primary, the story also has an unavoidable political component.
Riehl World View: I doubt very much he’d retire under the circumstances, evidently he wouldn’t need to. Her could simply hold onto the seat regardless of his health.
Michelle Malkin: First and foremost, keep him and his family in your thoughts and prayers. There’s already a rush to calculate the political consequences in the Senate.
Wizbang!: My prayers go out to Senator Johnson (D-SD). He’s so young, too, not even 60 yet. I hope he’s going to be ok.
Sensible Mom: And what if it was a very minor stroke caught early, in which case he will quickly make a full recovery? Won’t the media be embarrassed by their over-reaction if it had no impact on the balance of power? For goodness sake he’s a man with a family first and a politician second.
The Political Pit Bull: Obviously, news agencies are already talking about the potentially explosive political implications of this. For the record, I don’t object; it’s their job, after all. But for the meantime, I’m going to hold off on that here.
Sand in the Hourglass: Interesting…..You hate to gain politically at the expense of someone dying. My prayers are certainly with Sen. Johnson and his family.
Decision ’08: Well, look, I want control of the Senate as much as the next Republican, but not this way: best wishes for a speedy recovery to Senator Johnson. Sheer human decency should overrule partisanship on a story like this…
It isn’t a stroke. They’re calling it an undiagnosed illness at the moment. Hopefully, that’s good news.