There could be a coup if Labour does not win a critical by-election in Oldham tonight

There is a by-election to select a new MP in Oldham today and Labour is scared it is going to be punished by the voters. The election is being held in the constituency of Oldham West and Royton because Michael Meacher, the Labour MP who represented the area for 45 years, died in October. There are usually a small number of by-elections during a government's time in office and they are considered a test of the political mood among the public.

This is the first by-election since the general election in May and it couldn't have come at a worse time for Labour. The party has had who a disastrous couple of weeks leading up to Wednesday night's vote to bomb Syria. Labour's leader Jeremy Corbyn had been locked in a fight with his own party over the vote for several weeks. Corbyn, a committed pacifist, had initially wanted to force his MPs to vote against extending the RAF's bombing campaign from Iraq to Syria. He had to back down as it became clear that many in his shadow cabinet wouldn't vote with him and trying to dictate how they voted would lead to mass resignations.

The motion to bomb Syria was passed and embarrassingly for Corbyn, the biggest moment of the 10.5 hour debate before MPs voted, came when his own shadow foreign secretary Hilary Benn spoke out in favour of the motion. Benn's speech was so convincing that it could have persuaded 15 other Labour MPs to vote in favour of military action.

These open disagreements within Labour have made both Corbyn and his party seem weak and that's a problem. People often vote against parties they would normally support during by-elections in order to send a message that they are unhappy with the direction the party is going in. They do this because they know their vote won't change who is leading the country and, therefore, feel free to vote emotionally rather than rationally.

It should be easy for Labour to win in Oldham. Meacher won his seat in May with a huge majority of 14,738 - a big enough majority for it to assumed to be a "safe seat." But that's not happening.

The campaign has become a two-horse race between Labour's Jim McMahon and UKIP's John Bickley, someone with experience of winning votes from Labour supporters. Last year in Heywood and Middleton Bickley shocked Labour in a by-election, increasing UKIP's share of the vote by 36% and coming within 617 votes of taking the seat from them.

Bickley knows what he is doing and has been very successful at playing to the fears traditional Labour supporters have about the party's new leadership. Corbyn's pacifist tendencies are at odds with the majority of the population, and right from the start of the campaign, UKIP attacked Corbyn for his views on national security. Bickley even went as went as far as calling Corbyn a "clear and present danger" to the UK and UKIP voters have put up posters in their windows warning that Corbyn is a "security threat."

As Business Insider has reported before, there have been no public polls on voter intention in Oldham, but everything we know is indicating that Labour will lose a lot of votes. The most worrying thing for Labour is its own polling. Before the General Election, local Labour volunteers found that 32% of voters were planning to support Labour. However, someone from the party has leaked to the Telegraph that only half of those same voters are now planning to come back to Labour.

Corbyn might still retain lots of support among party members, but it seems traditional Labour voters are deeply suspicious of him. We know this because a number of MPs who have been campaigning in Oldham keep briefing journalists that Corbyn is costing them support. One told the Mirror "we should win but the majority will be nothing like it was for Meacher. A lot of that is down to Jeremy."

Labour's fear is reflected in the way it is running its campaign. Corbyn's picture has only been used once on its leaflets, while Corbyn himself has paid just one visit to Oldham.

If Labour loses a lot of votes in Oldham, it will be much more damaging to Corbyn personally then it will be for the party as a whole. Corbyn is currently able to tell Labour MPs who criticise him that he has a large mandate from Labour party members, something that annoys MPs who feel they have a much larger mandate from the millions of people who voted them in at the general election before Corbyn was leader. If Corbyn is seen as the reason that Labour does badly in Oldham, it will reinforce the view among many of these MPs that they hold the true mandate for power.

If Labour actually loses the seat, it will be even more serious for Corbyn. Plenty of Labour MPs will have watched Hilary Benn's speech and wished that someone like him was in charge of the party. A loss in Oldham would be the perfect excuse for them to take action.