The Working Families Party, which is allied with labor groups, exists in several states, though it a far more well known group in New York than anywhere else in the country.
In the Empire State, as in South Carolina, Democrats often compete for the Working Families line to increase the number of times they will appear on the ballot and, theoretically, the more votes they can get.
A cursory look at recent elections in South Carolina shows the Working Families Party line generally gets between 2 percent and 4 percent of the vote.
That might seem like a big deal in a close race. But keep in mind: Given the party's political leanings, its voters seem likely to vote Democratic if the "Working Families" option wasn't on the ballot. And while the party has gotten a decent number of votes in the races it has played in, it has generally only involved itself in downballot races in which the candidates aren't well-known. In a much-hyped special election, voters will be more familiar with the candidates and generally won't just casually mark a third-party line without knowing that they are essentially supporting the Democrat.
Updated at 3:21 p.m.