Another possibility is Herseth Sandlin, the former congresswoman who was unseated in one of 2010's highest-profile House races. Like Brendan Johnson, Herseth Sandlin also has a well-known last name: She is the granddaughter of two former statewide officeholders, former governor Ralph Herseth and former secretary of state Lorna Herseth.
Make no mistake: South Dakota was going to be a steep climb for Democrats before Johnson announced his retirement, and it remains a tough proposition. This is a state that Mitt Romney won by 18 points in 2012. And aside from Sen. Johnson, two Republicans represent South Dakota at the federal level.
The Great Plains states, and specifically the Dakotas, have been trending more and more Republican in recent years. But as 2012 showed, Democrats can still win statewide there. Now-Sen. Heidi Heitkamp (D-N.D.) won last year even as Romney carried her state by 20 points. Sen. Jon Tester (D-Mont.) won reelection even as Romney won his state by double-digits.
They both ran hyper-local campaigns and positioned themselves as centrists, something whomever Democrats nominate in Johnson's place will be expected to do as well.
On the one hand, not having to defend Johnson's votes for President Obama's stimulus package and health care bill -- two votes conservatives often target on the records of red state Democrats -- will provide Democrats with some relief, and possibly a better platform from which to run a hyper-local race. On the other hand, Democrats will be without the natural advantage of incumbency and having a well-known senator on the ticket.
And it's important to note that Republicans will still be expected to tie either Brendan Johnson or Herseth Sandlin to Obama. While Brendan Johnson has no votes on his record, Obama nominated him to his position. Herseth Sandlin, meanwhile, voted for the stimulus, though she voted against the health care reform measure.
There's a third name to watch in all of this, too: Rep. Kristi Noem (R), the woman who unseated Herseth Sandlin. While former governor Mike Rounds (R) had already launched a bid for Johnson's seat well before he retired, there has been speculation that Noem might also make a bid, setting up a competitive primary.
It would be a bold move for Noem to enter the mix, considering Rounds's political clout in the state. But until she says no, a competitive GOP primary could be in the former governor's future.
"She hasn’t ruled anything in or out," said Noem spokeswoman Courtney Heitkamp. "She just got through an election and is strictly focused right now on doing the job she was elected to do."
In other open Senate races, there has been considerably less clarity about the Democratic and Republican fields this early in the cycle. But in South Dakota, it would be surprising if neither Rounds of Noem ended up the Republican nominee and neither Johnson nor Herseth Sandlin ended up as the Democratic standard-bearer. That may well change as time goes on, but for now at least, that's where things stand.