Five candidates are contesting the West Tyrone by-election which was called after the resignation of Sinn Fein's Barry McElduff.
The former MP quit in January, 10 days after a controversy flared when he tweeted a video of himself with a Kingsmill-branded loaf on his head on the anniversary of the 1976 Kingsmill massacre. He insisted it was not an intentional reference to the murders of 10 Protestant workmen by republican paramilitaries, but he acknowledged he had caused hurt and offence to victims' families.
Sinn Fein candidate Orfhlaith Begley is defending a 10,000-plus majority in a seat where the party took just over 50% of votes cast in last year's general election.
Thomas Buchanan is contesting the seat for the DUP, Stephen Donnelly for the Alliance, Daniel McCrossan for the SDLP and Chris Smyth for the UUP.
Sinn Fein vice president Michelle O'Neill believes the furore over Mr McElduff’s Kingsmill loaf tweet will not affect Sinn Fein’s vote in the constituency. She said she expects a strong showing, portraying the poll as the first chance for voters to cast judgment on the breakdown of the Stormont talks process earlier this year.