The party has four MPs but has a long-standing abstentionist policy.

BBC: Sinn Fein gives evidence to Westminster committee

The SDLP's Patsy McGlone described the other candidates' decisions as "truly bizarre" and quipped that he had heard of abstentionist MPs before but never abstentionist campaigns.

BBC: Mid Ulster by-election: Two withdraw from TV debate

But, in contrast to Eastleigh, where the future of the Westminster coalition may be influenced by the outcome, Mid Ulster appears likely to see the replacement of one abstentionist Sinn Fein MP with another.

BBC: Promises easy to make in opposition