In a video, the former prime minister endorsed Ian Murray’s bid to replace Tom Watson as deputy leader.
Mr Murray, MP for Edinburgh South, also released endorsements from previous Labour heavyweights including ex-prime minister Gordon Brown, ex-chancellor Alistair Darling and former deputy leader Roy Hattersley.
Mr Blair said the party was facing a “huge challenge”, adding: “All of the candidates are wrestling with the inescapable fact that Labour’s lost four times in a row and that it has to be able to win power in order to bring about real change in the country.
“Ian understands that if Labour is to have any hope, it’s got to be able to win in every part of the UK. He gets it.”
Mr Blair, who won three elections, also said the party needs to have a “bold vision and programme” but warned it must be “realistic enough to win power”.
He added: “Only in that way can we bring about a government for the future and not return to being a protest movement of the past.”
Mr Brown said Mr Murray was the best candidate to take the party forward, while Mr Darling added: “The party needs to change to return to power and defeat SNP and Tory nationalism”.
Last night, the frontrunner in the deputy leadership race Angela Rayner spoke to ITV News about her mother’s “traumatic” battle with depression.
She appeared with her mother Lynn Bowen, who said: “I was in a dark place, she used to look after me. If it wasn’t for her I don’t think I’d be here today.”
The leadership ballot opened yesterday. Sir Keir Starmer, the frontrunner to replace Jeremy Corbyn, faces a battle for control of the party if he wins, according to Labour officials.
Allies of Sir Keir suspect the party’s general secretary Jennie Formby is recruiting officials before the new leader is appointed to entrench her power. A Labour source told The Times the appointments were routine and it was necessary to fill vacancies before the local elections in May.