Gordon Brown and Tony Blair have thrown their weight behind Ian Murray's bid to become Labour's deputy leader.
He is trailing behind rivals, including Angela Rayner, in terms of support from unions and local parties.
But Labour's last two prime ministers have endorsed his call for a complete "change in direction" for the party,
Mr Blair said Labour's only Scottish MP was making a "strong argument", while Mr Brown said he was inspired by his commitment to science and innovation.
To make it onto the final ballot, the five deputy leadership candidates need the backing of two unions and one affiliate group - representing at least 5% of the membership - or nominations from 33 constituency Labour parties (CLPs).
So far, Ms Rayner is the only candidate to have done so, having got the backing of the Unison, GMB and Usdaw unions.
The other candidates - Mr Murray, shadow equalities minister Dawn Butler, Tooting MP Rosena Allin-Khan and shadow justice secretary Richard Burgon - have until 14 February to pass this threshold.
Mr Burgon is closest, with two unions - Unite and BFAWU - in his corner.
It emerged on Tuesday that Labour's membership has gone up by 114,000 since its defeat in December's general election.
Another 14,700 people have paid £25 to be "registered supporters", allowing them to vote in the leadership and deputy leadership ballots, the result of which will be announced on 4 April.
Mr Murray has been the most critical of the candidates of Jeremy Corbyn's leadership and Labour's election campaign. Others have suggested its defeat was down to poor communication and strategy - and the Brexit factor - rather than its policies and personnel.
In an article for the Times' Red Box, he said activists should not vote for him if they thought the party "won the argument" at the election.
"We can't just continue with a different voice and a different face; we need to change direction too," he wrote.
In response, Mr Blair - who won three general elections as leader - called on people to listen to what Mr Murray had to say.
“We can’t just continue with a different voice and a different face; we need to change direction too."— Tony Blair Institute (@InstituteGC) January 29, 2020
Strong argument by @IanMurrayMP that needs to be made – and heard – if Labour is going to once again become a party of government -Tony Blair. https://t.co/Y6BrQjBUEZ
In the past few days, Mr Murray has won the backing of the Labour Movement for Europe.
Mr Brown said he was proud to back his fellow Scot to succeed Tom Watson as deputy leader.
"I know Ian Murray to have been committed to and directly involved in supporting investment in science in our country," he said.
"I think that as a candidate whom I support for the deputy leadership he can ensure that the next Labour manifesto will give the priority to science and innovation that scientists deserve and is the best way forward for our country."
Mr Murray welcomed the duo's support, saying the governments they led between 1997 and 2010 had been "transformational" and they had changed the country by first changing Labour.
"The only way to get Labour back into power is to change our party once again," he said. "If members are happy with remaining in opposition and want more of the same, they shouldn't vote for me - they should vote for a continuity candidate.
Both Mr Blair and Mr Brown have, so far, declined to be drawn on who they are backing to be leader. Only shadow Brexit secretary Sir Keir Starmer and Wigan MP Lisa Nandy have made it on to the ballot so far.
Shadow business secretary, Rebecca Long-Bailey, has the backing of the Unite union and 24 CLPs, while the shadow foreign secretary, Ms Thornberry, has four CLPs behind her.
Correction 30 January 2020: An earlier version of this story incorrectly stated Ian Murray had received the endorsement of Scientists for Labour, which Gordon Brown is patron of.