Jeremy Corbyn has appeared in a campaign video for Rebecca Long-Bailey, in an apparent show of support for her bid to replace him as Labour leader.
In the video, posted on Twitter, Mr Corbyn promised he would give the shadow business secretary his "absolute support" if she wins the contest.
He has not appeared in campaign videos for her leadership rivals, Sir Keir Starmer and Lisa Nandy.
Members began voting earlier this week, with the result announced on 4 April.
Mr Corbyn has also appeared in a campaign video for shadow justice secretary Richard Burgon, one of five candidates for the deputy leader position.
In the video, he said there was "a lot of merit" in Mr Burgon's proposal to give party members a vote on whether a future Labour government should take part in military action, in certain circumstances.
Mr Corbyn did not offer an explicit endorsement of either Mrs Long-Bailey or Mr Burgon in his appearances in their respective campaign videos.
He said last month that he would not be revealing which of his potential successors he would be voting for.
Asked in the video which issues she thinks will come up at the next election, Mrs Long-Bailey replied: "I think the next general election will be a climate election."
She adds that it will be important to make Labour's plans for a "green industrial revolution" a "central part" of the party's offer to voters.
Last week Mr Corbyn, who is standing down after the party's heavy election defeat, said he would consider serving in the shadow cabinet if offered a job by his successor.
Some of those close to Mr Corbyn fear his political legacy is looking shaky and are keen to shore up Mrs Long-Bailey's support to ensure - at the very least - she has a prominent frontbench role.
But how worried should he be about that legacy?
A YouGov poll suggested that even if Mr Corbyn himself were a candidate in this party election, he would get 28% of members' first-preference votes, while Sir Keir would garner 40%.
The unknown question is just how far and how quickly Sir Keir would depart from the Corbyn agenda if he wins.
Mrs Long-Bailey has said she would offer him a place in her top team if she is successful, and has previously scored his leadership of the party as "10 out of 10".
Her candidacy has been backed by key allies of Mr Corbyn, such as shadow chancellor John McDonnell and shadow home secretary Diane Abbott.
In an interview with the i news website on Thursday, Mrs Long-Bailey sought to distance herself from a charge made by her critics she is the "continuity Corbyn" candidate.
"If I'm a continuity socialist, then fine I will accept that title, but there is no such thing as Corbynism."
The leader of the Unite union, which has given more than £200,000 to Mrs Long-Bailey's campaign, said she was best placed to defend and build on the "achievements" of the Corbyn era.
In an article for Labour List, Len McCluskey praised her "loyalty" to Mr Corbyn and urged Labour members to ignore the "siren voices" of those who wanted to turn the clock back to a time before Mr Corbyn was leader.
While Mrs Long-Bailey would have "her own priorities and do things her own way", he said he was confident her approach of "continuity socialism" would connect with "natural Labour" supporters.
But he added: "Ultimately it's up to Becky to show that she can inject the magic back into our party. And if she can do that, then she will win."
Mrs Long-Bailey revealed earlier this week she had received £120,000 in donations from Momentum, the campaign group which supported Mr Corbyn's bid for the party leadership in 2015.
Wigan MP Ms Nandy has disclosed a series of recent donations on her website, the biggest of which was £25,000 from the GMB trade union.
The Young Labour national committee, which is backing Mrs Long-Bailey for the leadership, wrote to all leader and deputy leader candidates on Thursday, urging them to publish all donations to their campaigns to ensure party members are fully informed about who is bankrolling them.
All candidates, including Sir Keir, the party's Brexit spokesman, have been declaring support in the Commons Register of Members' Interests.
By 10 February, when the register was last updated, Sir Keir had recorded a series of small donations from Unison, a union backing his campaign, totalling just over £8,000.
The most recent recorded donation he has registered was accepted on 24 January. He has not posted any more recent donations on his website.
A spokesman for his campaign said a more recent batch of donations had been submitted to Parliament and they expected it to be published next week.