Copeland and Stoke-on-Trent Central by-elections will be decided by turnout

The Labour Party faces two awkward by-election contests this Thursday, each caused by the resignations of sitting MPs.

Both Jamie Reed in Copeland and Tristram Hunt in Stoke-on-Trent Central knew that a deeply divided party would struggle to retain them.

Normally the Opposition, particularly at this stage of the electoral cycle, is firmly on the front foot - buoyed by their own supporters and boosted by protest voters.

Therefore defending these constituencies, held for decades by Labour, should be a formality.

That they are not is a measure of how far and how quickly Labour's traditional electoral base is eroding.

Its collapse in Scotland, where it lost all but one of its 41 seats at the last election, resulted from the Scottish nationalists skilfully riding the wave of separatist thinking kicked off by the independence referendum.

The EU referendum in 2016 exposed further fault lines.

The strongest areas favouring Brexit and Remain are both dominated by Labour voters.

Labour struggles to find a message that simultaneously resonates with voters across London and with those suffering from de-industrialisation in the Midlands and North of England.

Mr Corbyn has received not one but two ringing endorsements from the party's voting members, yet national polling tells a different story where the Conservatives hold a comfortable lead.

Ipsos/MORI found almost two in every three voters were dissatisfied with Mr Corbyn as Opposition leader, a larger number than that recorded for Michael Foot.

More than 80 Labour MPs at some point have been members of the shadow cabinet since he took over.

These by-elections are both a threat and an opportunity.

Win them both and Mr Corbyn's critics will be momentarily silenced, unless more of his MPs choose job offers rather than waiting to be de-selected once the new parliamentary boundaries are finalised.

Losing Copeland, where the Conservatives pose the greatest threat, means Labour would experience the rare embarrassment of an opposition party losing a by-election to the Government.

The last occasion was in 1982.

Recent national polling indicates a tight race for the seat, with Labour's majority of 2,500 votes vulnerable.

In local elections held on the same day in May 2015, Conservative candidates across the constituency out-polled Labour.

A low turnout here would point to a Labour defeat.

The winner's margin may only be a few hundred votes.

Defeat in Stoke-on-Trent Central, where UKIP is the main challenger, would be a much greater setback for Labour to bear.

The form guide suggests 'Labour hold' in a constituency it retained by more than 5,000 votes just two years ago.

Labour may be struggling in the opinion polls, but UKIP has no momentum either.

In local council by-elections around the country it is regularly eclipsed by the Liberal Democrats as the host for protest voters.

But there is something about this seat that persuaded UKIP's new leader Paul Nuttall to throw his hat into the ring, despite a clear risk that failure to win it would place both his judgement and his party's future under the spotlight.

That something is the set of circumstances that led an estimated two-thirds of the constituency's voters to support Brexit.

This is classic 'left-behind' territory: the closures of mines and steelworks followed by a serious decline in ceramics greatly weakening the local economy and leaving what was a highly skilled workforce with fewer jobs.

Given its stated electoral intention to replace Labour, it became a case of "put up or shut up" for UKIP once the vacancy arose in one of the country's most socially deprived constituencies.

However, there remains a naivety about UKIP's campaigning - beginning with Mr Nuttall's contrivance to declare a postal address that lay within the constituency, and the more damaging claims regarding his direct connections with victims of the Hillsborough disaster.

Such mishaps would have scuppered his chances were it not for the indiscreet online postings of his Labour opponent, whose tirade against strong-minded women is unlikely to endear him to half of the voters - not to mention his curt dismissal of Brexit and, by implication, all those who support it.

Two key factors will determine the outcome in Stoke-on-Trent Central.

Firstly, there is a significant Conservative vote - more than 7,000 votes - which may remain loyal but could tip the balance if sufficient numbers decide to go for the strategic option of voting UKIP to oust Labour.

Secondly, voter mobilisation is critical in a constituency that recorded the lowest turnout (51%) at the last general election.

Were it not for the publicity surrounding this contest, a turnout of around 20% might be regarded as the norm.

But with the stakes so high for both Labour and UKIP, the result hinges on which party is best positioned to persuade its supporters that voting will make a difference.

:: Candidates in the Copeland by-election
Michael Guest - Independent; Rebecca Hanson - Liberal Democrats; Trudy Harrison - Conservatives; Roy Ivinson - Independent; Jack Lenox - Green Party; Fiona Mills - UKIP; Gillian Troughton - Labour.

:: Candidates in the Stoke-on-Trent Central by-election
Mohammed Yaqub Akram - Independent; Zulfiqar Ali - Liberal Democrats; Jack Brereton - Conservatives; The Incredible Flying Brick - Official Monster Raving Loony Party; Adam Colclough - Green Party; Godfrey Davies - Christian Peoples Alliance; Barbara Fielding - Independent; David Furness - British National Party; Paul Nuttall - UKIP; Gareth Snell - Labour.