Should Johnson be away from the Chamber for a key vote, then yes - it's an issue ... that is if it's a straight party-line vote and every Republican shows up and the Independents stay away. That's because the Democratic Party has a majority only by virtue of two Senators who are Independents: there are 49 Ds and 49 Rs. This scenario, however, is highly unlikely.
However, if Johnson were to die or to resign, then South Dakota Governor Mike Rounds (R) would appoint a replacement, following South Dakota state law. Assuming he appointed a fellow Republican, then the Rs would control the Senate (because a 50-50 tie goes to the party with the Vice President).
When President Bush was sworn into office in January 2001, the Senate was split 50-50. At that time, the parties agreed to have equal numbers on committees; Republicans appointed the committee chairmen. However, in June, Vermont Sen. Jim Jeffords abandoned the Republican party and elected to caucus with Democrats as an Independent. Thus Democrats assumed control of the Senate.
Illness Not A Cause For Dismissal
An ill Senator does not automatically mean a vacated seat, as South Dakotans are aware. In 1969, the Republican Senator from South Dakota, Karl Mundt, suffered a stroke from which he did not fully recover. Nevertheless, he held office for three more years, until his death.
Republicans, then numbering 45 in the 100-member Senate, waited for two years before stripping Mundt of his seniority and committee assignments.
Sen. Strom Thurmond (R-SC) made news by turning 100 while in office. But he "was so frail at the end of his career that he could hardly walk onto the Senate floor without gripping the arms of staff members."
More recently, Sen. Joe Biden (D-DE) missed seven months in 1988, and Sen. Jay Rockefeller (D-W.VA) missed three months this year. Both were on what we might call "medical leave" -- Rockefeller for his back and Biden, like Johnson, because of brain surgery.
Elected in 1996
Johnson defeated an "entrenched" incumbent in 1996 to capture his Senate seat. He held on to the seat in 2002, barely defeating his Republican opponent, Rep. John Thune. (South Dakota's only Representative is currently a Democrat.) Johnson won with 49.62 percent of the vote; Thune took 49.47 percent.
Ironically, it is the Senate -- not the voters or Governor of South Dakota -- that has the authority to force Johnson to resign. However, this rarely happens.
So, yes, the Democrats could lose control of the Senate. But with a 51-49 margin, they've known that from the get-go.
I shake my head at the alarmist nature of the headlines and news frame. According to The New York Times a spokesman for Rep. Stephanie Herseth (D-SD) said: "South Dakotans find all this talk of succession to be completely unnecessary and distasteful at this point."
So do I. Best wishes, Sen. Johnson, for a full recovery.