The second largest by-election ever held in the UK takes place today.

Voters across South Yorkshire have the chance to choose a new police and crime commissioner, following the resignation in September of Shaun Wright in the wake of the Rotherham child sex abuse scandal.

The total number eligible to vote is 1,008,967. Just how many of these people choose to cast their ballot will be one of the big stories of the poll. Turnout at the previous contest in 2012 was a miserable 14.5%. Today’s figure will almost certainly be lower. It may even dip below the all-time low of 10.2% seen in the West Midlands PCC by-election in August.

Such a pathetic turnout would supply more ammunition to Labour and the Liberal Democrats, both of whom have said they would scrap PCCs if they win the general election.

The other big story of the by-election will be the identity of the winner. Or more precisely, how small a margin of victory the winner will have over the runner-up.

Labour should win, despite some loose talk in the media over the past few days of Ukip being in with a chance. But an exceptionally low turnout might mean the result will be close.

Labour ought to benefit from the absence of the Lib Dems, who aren’t bothering to field a candidate. Instead, voters are being presented with a line-up of four hopefuls, three of whom are of the right:

• David Allen (English Democrats)
• Alan Billings (Labour)
• Jack Clarkson (Ukip)
• Ian Walker (Conservatives)

David Allen is contesting the post for the second time. In 2012 he actually came second, beating the Tories, Ukip and the Liberal Democrats:

The presence of the English Democrats on the ballot paper will probably split the anti-mainstream vote. Were this a fight between solely Labour and Ukip, I wouldn’t be so sure that Labour would win.

But there is a further factor that may shape the outcome. The result will be decided using the supplementary vote (SV) system. This means that if nobody wins more than half the vote on the first count, all candidates bar the top two get eliminated and their alternative preferences allocated to the survivors. Whoever emerges with the most number of votes after this round is declared the winner.

To whom might Ukip supporters give their second preference? May the party actually benefit from being the second choice of the Tories and/or English Democrats? It’s possible, but here again the three-way split of right-wing support may help Labour over the finishing line.

South Yorkshire covers Barnsley, Doncaster, Rotherham and Sheffield. Ukip will be hoping to do well in some of the suburbs, but Labour ought to be stronger in the metropolitan centres – particularly the more multicultural areas.

Polls close at 10pm. The result of the first round of voting is expected on Friday at around 2pm, but could be later if there is a recount.