Jeremy Corbyn is facing his toughest electoral test to date as voters go to the polls in two crunch by-elections in Labour's traditional heartlands.
The party is fighting to hold Copeland and Stoke-on-Trent Central in the face of strong challenges from the Conservatives and Ukip.
The contests were triggered by the resignations of the sitting Labour MPs Jamie Reed and Tristram Hunt - who quit to become director of the Victoria and Albert Museum.
With the party trailing badly in the opinion polls, they are being seen as a test of whether Mr Corbyn can re-connect with its traditional supporters in the wake of the Brexit referendum vote.
There have been signs in recent days of growing confidence among Labour MPs that they will hang on in Stoke Central, despite a concerted challenge by Ukip.
But in the Cumbrian seat of Copeland, the bookies still make the Conservatives the narrow favourites to snatch victory.
A Tory win would mark a rare by-election gain for a Government party, underlining the scale of the task facing Mr Corbyn in his quest for Downing Street.
The seat has been Labour since it was formed in 1983, but its majority has been steadily eroded by the Conservatives in subsequent elections, dropping to just 2,564 in 2015.
The constituency is heavily dependent economically on the Sellafield nuclear processing facility and Mr Corbyn's past opposition to nuclear power has not gone down well.
Labour has sought to counter with a hard-hitting campaign focusing on possible cuts to maternity services at the West Cumberland Hospital, with a claim local midwives believed it would lead to "mothers and babies dying".
In Stoke Central, which Labour has also held since it was created in 1950, the main challenge is expected to come from Ukip which finished second in the 2015 general election, trailing by 5,179 votes.
The city voted overwhelmingly for Leave in the referendum - earning the nickname "Brexit Central" - and the party believes it is potentially fertile ground for an electoral breakthrough, putting up its leader, Paul Nuttall, as candidate.
However, it has been hit by a series of setbacks, with Liverpudlian Mr Nuttall being forced to apologise for a false claim on his website that he lost "close friends" in the Hillsborough disaster.
He vehemently denied a press report questioning his claim to have attended the match as a 12-year-old Liverpool fan saying he had been the victim of an "almost evil smear campaign".
However, Ukip faced further difficulties when wealthy backer Arron Banks tweeted he was "sick and tired" of hearing about Hillsborough, prompting two senior Ukip officers in the city to quit citing the party's "crass insensitivity" on the issue.
Labour candidate Gareth Snell has also had his difficulties, making a rocky start to the campaign when it was disclosed that he had described Mr Corbyn as an ''IRA-supporting friend of Hamas" and called Brexit a "massive pile of shit".
He also was forced to apologise for disparaging remarks about women on TV he had made on Twitter, including calling the presenters of Loose Women "squabbling sour-faced ladies".