Jeremy Corbyn will not have to consider his future as Labour leader if his party loses both upcoming by-elections, a shadow cabinet ally has said.
Shadow home secretary Diane Abbott said the Opposition is "hopeful" of retaining its seats for Copeland and Stoke-on-Trent Central on Thursday.
But she dismissed the prospect of leadership change in the event of Labour losing either or both contests.
Mr Corbyn is facing his most difficult electoral challenge so far with Ukip expected to pose the greatest challenge in Stoke, particularly given the Midlands city's support for Brexit in last year's EU referendum.
But Ukip leader Paul Nuttall, who is standing for the seat, has faced criticism following claims over inaccuracies on his website.
The Conservatives are pushing to make a rare by-election gain for a governing party in Copeland, Cumbria.
Ms Abbott, speaking on Sky News' Sophy Ridge on Sunday, said: "These are difficult by-elections, they're going to be quite tight.
"We are hopeful of winning both of them but it's really important that every Labour supporter comes out to vote.
"And if we lose one or we lose both, I think the party will go forward. It has to go forward."
Presenter Ms Ridge pressed Ms Abbott: "So no questions over Jeremy Corbyn's leadership then if he loses?"
Ms Abbott replied: "No, no. I mean there are people who have been opposed to Jeremy from the very beginning, I'm not one of them and I think we have to move forward."
The resignations of sitting Labour MPs Jamie Reed and Tristram Hunt led to the by-elections being called.
Ukip's only current MP Douglas Carswell suggested victory in Stoke could be a turning point in British politics, similar to when Labour began ousting the Liberals in the early 20th century.
"It's important, it could turn out to be like the Spen Valley by-election in 1919 when an established old party was finally seen off and defeated by the new insurgents - the only difference being that Labour is no longer the insurgent party, it's the old established party," he told BBC One's Andrew Marr Show.
"If we win this, like that by-election almost a hundred years ago, it could mark the turning point at which Labour is displaced. And if it did, that would be wonderful."
Mr Carswell added: "We are still the underdog and we are up against a party which has been running Stoke since 1950.
"But they had a 20,000 majority in that constituency in the late 1990s, it's pretty thin now. You only need about 8,000 or 9,000 votes to win the by-election, I think and I think we can just about do it."
Shadow justice secretary Richard Burgon said Copeland is not a "safe Labour seat" and predicted "tough contests in both by-elections.
"I think those two by-elections are not going to be easy by-elections, contrary to some reports Copeland is not a so-called safe Labour seat," he told BBC Radio 5 Live's Pienaar's Politics.
"I haven't written it off, I believe that Labour can win both of these by-elections. But I think it will be tough, I believe they will probably be close, I believe that every vote will count.
"But Copeland is not a so-called safe Labour seat, it has been held - by and large - mostly by small majorities and then there are other challenges in Stoke."
He added: "It is never welcome to lose any election and people may be surprised to learn that the majority in Stoke at the last general election was only 5,000."