The result in the West Tyrone by-election isn't expected until the early hours of Friday as voters across the constituency go to the polls today.
Sinn Fein is the odds-on favourite to retain the seat, with 26-year-old solicitor Orfhlaith Begley on course to become the area's first female MP.
The by-election follows the resignation of Barry McElduff four months ago. He stood aside amidst fury over a video he posted on social media of himself with a loaf of Kingsmill bread on his head (below) on the anniversary of the Kingmill massacre.
Polling stations across West Tyrone opened at 7am and will close at 10pm.
The Electoral Office for Northern Ireland said it expected the result around 3.30-4am, although the candidates are hoping that the declaration will be earlier.
The campaign has been low-key, although Sinn Fein president Mary Lou McDonald and the other party leaders have all joined their candidates on the campaign trail.
The five main parties are contesting the election. The candidates include local DUP MLA Thomas Buchanan, SDLP MLA Daniel McCrossan, Ulster Unionist councillor Chris Smyth, and Alliance's Stephen Donnelly.
Sinn Fein secured 51% of the vote in last year's Westminster election. Mr McElduff came in more than 10,000 votes ahead of his nearest rival, Mr Buchanan. Turnout, which was 68% last June, is expected to be lower today.
Former Sinn Fein vice-president Pat Doherty won the seat from Ulster Unionist Willie Thompson in 2001.
Sinn Fein's opponents have focused on the party's boycott of Westminster, with the SDLP arguing that an abstentionist MP cannot deliver a strong anti-Brexit message.
Mr McElduff, who had been a Sinn Fein MLA for 19 years, was MP for only seven months before his resignation.
He posted the Kingsmill bread video on January 5, the 42nd anniversary of the massacre in which 10 Protestant workmen were shot dead by the IRA. It was seen by many as a deliberate insult to the victims and their families, although Mr McElduff denied that and said he meant no offence and the timing of the video was a ghastly coincidence.
In a statement yesterday, Mr Buchanan said voters had the chance to "pass judgment at the polls on Sinn Fein disrespect for victims and 17 years of no representation in the House of Commons".
He said: "I have pledged on the doors to be a voice for everyone in West Tyrone in Parliament. People who were never before DUP voters have spoken to me about their disgust for how innocent victims were mocked and disrespected by Sinn Fein. Many have said they were embarrassed that such comments were associated with West Tyrone.
"Orfhlaith Begley has failed to condemn Barry McElduff's outrageous video which mocked the Kingsmill victims on the anniversary of the atrocity.
"Indeed, she also opted not to condemn the naming of a playground after Raymond McCreesh, who was arrested in possession of a gun used at Kingsmill.
"Many will question what's the difference between Orfhlaith Begley and Barry McElduff?"
Mr Buchanan added: "The DUP's delivery from Westminster for roads, schools, hospitals and broadband has demonstrated that it matters when MPs take their seats. Sinn Fein has wasted West Tyrone's influence in Parliament for the last 17 years. They claim the expenses but don't take their seats. People are tired of such an approach."
Sinn Fein vice-president Michelle O'Neill lauded Ms Begley as "a young, dynamic advocate, in her political activism and work as a solicitor, for rights and equality and for a new future in a new agreed united Ireland".
Mrs O'Neill said: "She has identified the twin threats of Brexit and relentless Tory austerity as the biggest threats facing the people of West Tyrone.
"The people of West Tyrone will have a fighter for them to stand up against these threats."