Liz Kendall has become the first Labour MP to declare that she will seek the party leadership as she joined a chorus of criticism of the tactics that ended in a crushing General Election defeat.

The shadow health minister conceded it would be "phenomenally difficult" to sufficiently rebuild the party around a more attractive pitch to voters in time to defeat the Conservatives in 2020.

Potential rival Chuka Umunna earlier said he would "play the fullest part I can" but declined to confirm that he would join the race to succeed Ed Miliband, who resigned after leading the party to a shocking defeat at the polls.

Both joined calls by senior Blairites - including ex-PM Tony Blair himself - for the party to offer a more "aspirational" message that appealed beyond the traditional working class vote.

It was no good being a government that thought the answer was "doing things to people or for them", she said, and instead needed to ensure everyone had the opportunity to succeed for themselves.

"It is not just enough to critique what is going on; you have to set out something people can believe in", she told BBC1's Sunday Politics, suggesting doubts over Mr Miliband's leadership were supressed because Labour believed in "total loyalty".

Mr Umunna said it was "too early" to declare his hand, with the party's ruling body due to meet this week to decide the timetable for the contest.

But asked if she was standing, Ms Kendall told the programme: "Yes".

Mr Umunna said Mr Miliband led a "courageous" campaign and accepted part-responsibility for failing to prevent a strategy focused on Labour's "core vote" but rejected claims the scale of the defeat meant it could not return to power in 2020.

"I certainly intend to play the fullest part I can in rebuilding our party and having the proper debate that we now need to have to make sure we win," he told BBC1's Andrew Marr Show when pressed to say if he would stand.

"We are down but we are not out. The scale of the defeat is of '92 proportions but there is no reason why we shouldn't get back in in 2020. I do not buy this idea that this is somehow a 10-year rebuilding project.

"We can do this in five years if we make the right decisions now and present that aspirational and compassionate case to the British people which we are so good at. We can do this.

"We cannot have a message that anybody is too rich or too poor to be a part of our party. What the Labour Party does well is build a big tent of people of different backgrounds, creeds, colours, races, religions, economic circumstances.

"And it is when we have an offer that is big tent and appeals to a lot of people, that's when we win."

It came as former New Labour strategy chief Lord Mandelson accused Mr Miliband of throwing away a genuine chance of returning the party to power by his "terrible mistake" of shifting the party to the left.

He said the party leader had delivered "a passionate and a professional performance" in the campaign but had seriously misjudged the public mood.

"The awful, shocking thing about this election is that Labour could have won it or at least come a very near second. The reason we lost it, and lost it so badly, was because in 2010 we discarded New Labour rather than revitalising it and re-energising it and making it relevant for new times that we faced.

"That was a terrible mistake.

"Literally we were sent out and told to make an argument - if you can call it an argument - which basically said 'we are for the poor, we hate the rich' , ignoring completely the vast swathe of the population who exist in between who do have values like ours."

Asked what was missing from the Labour manifesto, the former business secretary said: "An economic policy."

Mr Blair, who led the party to three consecutive election victories, said the party had to show that it stood for "ambition and aspiration" as well as compassion and care.

Shadow education secretary Tristram Hunt said he is "definitely thinking about" running for Labour leader.

He also warned Labour is "in a hole" in Scotland and England and needed to appeal to traditional Labour communities plus the aspirational "John Lewis couple".

The Stoke-on-Trent Central MP told BBC Radio 5 Live's Pienaar's Politics:"The reason why this debate needs to be long and deep and painful for the Labour Party is we are in a real hole.

"We are in a hole in Scotland and we are in a hole in England and we've got challenges in Wales as well.

"But the issue in England is this double bind of losing traditional Labour communities often under pressure from Ukip, and not speaking to an aspirational, John Lewis couple who we should be on their side."

There were calls for a lengthy leadership campaign - extending even into the party's annual conference in September.

MP Jonathan Ashworth - a member of the NEC - wrote on Twitter: "We need a leadership contest that tests all contenders.

"I'm pushing for a long contest that allows all to give speeches at conference."

General secretary of the GMB union Paul Kenny told Sunday Politics: "We need a really decent, long, thought-through process because I agree with some of the candidates that it's not necessarily the figurehead we need, we need to make sure we've got the right cargo on board."

"The message about what Labour stands for - giving advantages in life and opportunities in life - was lost in the election campaign."

But in a message to veteran Blairites calling for a swing back towards the centre ground, he said: "Clearly a return to New Labour or anything like it would hardly win back Scotland."

GMB general secretary Paul Kenny said Labour should now take time to hold a "serious examination" of what happened in the election rather than rush into making any decisions.

He told the Press Association: "Any attempt to reintroduce a quick fix based on nostalgia and a world as it was 25 years ago would lead to disaster.

"The absurd anti-union remarks made by Lord Mandelson will do nothing but reignite arguments and divisions which are completely unconnected to why Labour lost."

Mr Kenny said a return to New Labour would not help the party recover, especially in Scotland.