Usually when an elected official retires it's bad news for that person's party, but R.C. Soles' decision not to seek another term is unambiguously good news for Democrats.
Polling data released by the Republican Senate caucus yesterday showed Soles trailing his potential GOP foes by 24-26 points. There is no doubt that if he had made another run for it he would have been defeated. It's still going to be a tough hold for Democrats, but you have to look at it as a toss up now rather than a strong Republican advantage.
It's a very similar situation to what the GOP faced with Jim Bunning in Kentucky earlier this year. Even though it's a Republican leaning state, his personal issues were going to let a Democrat win if he had decided to run for reelection. Once he decided not to run the seat went immediately back to leaning toward the GOP. Soles' district is a little more Republican than the state as a whole but it's still ancestrally Democratic and has a large Democratic registration advantage- the party has a much better chance of holding onto this seat now than it did a week ago. It will doubtless be one of the most competitive races in the state.
When an incumbent has as much baggage as Soles it produces exceptions to the rule that retirements are bad for the departing official's party.