Four candidates were successfully nominated to stand in the election: Andy Burnham, Yvette Cooper, Jeremy Corbyn, and Liz Kendall.

Support for Corbyn, who entered the race as a dark horse candidate,[2] and the release of opinion polls which showed him leading the race, led to high-profile interventions by a number of prominent Labour figures including Gordon Brown,[3] Tony Blair,[4] Jack Straw,[5] David Miliband,[6] and Alastair Campbell, among others,[7] many of whom argued that Corbyn's election as leader would leave the party unelectable.

[14] The deadline on 12 August 2015 to join as a member or supporter was extended by 3 hours due to heavy demand making the party website difficult to use.

Public nominations for candidates by MPs were as follows:[24] Before dropping out of the race on 12 June, Mary Creagh had 10 nominations: Sarah Champion, Jo Cox, Neil Coyle, Thangam Debbonaire, Helen Hayes, Susan Jones, Mike Kane, Stephen Kinnock, Tulip Siddiq[24] A total of 26 Labour MPs did not nominate any candidate: Graham Allen, Ben Bradshaw, Alan Campbell, Rosie Cooper, Mary Creagh, Stella Creasy, John Cryer, Angela Eagle, Natascha Engel, Caroline Flint, Barry Gardiner, Roger Godsiff, Harriet Harman, Meg Hillier, Lindsay Hoyle, Alan Johnson, Gerald Kaufman, Ed Miliband, Rob Marris, Ian Murray, Graham Stringer, Mark Tami, Keith Vaz, Tom Watson, David Winnick, Rosie Winterton Burnham was praised for having both "a radical left-wing vision" and being credible enough "to unite the party and win back power",[55] as well as for being someone who "actually listens to party members and the public".

[56] Burnham attracted press criticism for claiming £17,000 a year from the taxpayer to rent a London flat, despite owning another within walking distance of the House of Commons.

A spokesman for Burnham said that renting out the original flat was necessary to "cover his costs" as parliamentary rule changes meant he was no longer able to claim for mortgage interest expenses.

[62] Owen Jones argued in The Guardian that the reason Corbyn was so popular was because he "offers a coherent, inspiring and, crucially, a hopeful vision" addressing social injustice and economic inequality, comparing the surge of support for Corbyn to the popularity of both UKIP in England and the Scottish National Party (SNP) in Scotland.

[65][66][67] However, the Huffington Post criticised her, saying "Liz Kendall just doesn't seem to have it, she seems to be always on the verge of tripping over her own words, as if she is perpetually being caught off guard.

[76] Leadership candidates Andy Burnham and Liz Kendall also responded by saying there was no evidence of infiltration, and while not dismissing the claims, Corbyn stated he only wanted "genuine Labour supporters" to vote for him.

[77][78] It emerged in early August 2015 that 260 former candidates from the Green Party, Left Unity and the Trade Unionist and Socialist Coalition had attempted to become registered supporters but would now be blocked from voting.

[79] Shortly before this it was revealed that Conservative MP and former junior minister Tim Loughton had been caught applying to become a registered Labour supporter, subsequently claiming that his intention was to "blow the gaff on what a complete farce the whole thing is".

[80] Veteran Labour MP Barry Sheerman also joined calls for the election to be "paused" over the fears of infiltration by other parties.

[124] Burnham later replaced Yvette Cooper as Shadow Home Secretary after she announced that she would not serve on a Labour frontbench led by Corbyn and that she would instead focus on playing a role in the upcoming EU membership referendum.

[127] During the announcement of Corbyn's victory, Jamie Reed resigned as a Shadow Health Minister, citing his difference in opinion, with the new leader, over the replacement of the Trident missile system.

[130] This echoed the same warning from other Cabinet ministers, including Michael Fallon, then Secretary of State for Defence,[131] and from an email sent by Conservative Campaign Headquarters to party members.

However Nick Hurd, former Minister for Civil Society, and Daniel Hannan, then MEP for South East England, both warned their party against "complacency".

[132] Tim Farron, then leader of the Liberal Democrats, commented on the "massive space in the centre ground of British politics" that had been opened up, due to the perceived shift leftwards by Labour, and suggested that the Liberal Democrats would be able to appeal more directly to "sensible, moderate, progressives who are opposed to what the Conservatives are doing, but cannot bring themselves to support a party of the hard left".

[133] The President of the Liberal Democrats, Sal Brinton, accused Labour of abdicating "its responsibilities" as a party of effective opposition.

Willie Rennie, leader of the Scottish Liberal Democrats, said that Corbyn's success meant a "return to the damaging see saw politics of the past".

[134] Kirsty Williams, leader of the Welsh Liberal Democrats, attacked Corbyn as the "opposite of what this country needs", accusing Labour of returning to its 1970s policies.

As Krugman noted, not only the candidates but also the Labour moderates implicitly agreed with their opponents' idea that the government budget should always be balanced by the austerity policies.

[146] Krugman argued that it was a false claim that the Labour party, in power during 1997–2010, spent far beyond their means and caused a debt crisis.