Democrats won a 51-49 majority in the November election. South Dakota's governor, who would appoint any temporary replacement, is a Republican.
If the two-term senator, 59, is unable to serve when the 110th Congress convenes Jan. 4, South Dakota Gov. Mike Rounds would appoint a replacement. Johnson is up for re-election in 2008.
South Dakota Secretary of State Chris Nelson said there are no special restrictions on such an appointment and a replacement would not have to be from the same political party.
Hopefully, Johnson will be fine and will live to serve many more years in the senate. Best wishes to him and his family.
Update: CNN reports that Johnson's spokesperson says it was not a stroke or a heart attack. They're still not sure what happened though.