Neil Kinnock has joined a concerted fightback on behalf of the Labour leader Ed Miliband, attacking the "vindictiveness" of a hostile media, as other senior party figures warned that without more unity Labour would be out of power for a generation.

Kinnock, who while Labour leader faced his own bloody battles with the press in the 1980s and early 90s, claimed Miliband was being aggressively targeted but insisted he had the courage and conviction to survive and lead Labour to victory in next year's general election.

After another torrid week for the Labour leader, which saw him suffer dire personal poll ratings and thinly veiled criticism from senior figures including Lord Mandelson, Kinnock said part of the reason the rightwing press was going for Miliband was because it knew he was bold and a threat to Tory victory hopes.

Kinnock, who backed Ed over his brother, David, in the 2010 leadership contest, said: "A hostile press which thought he was a soft target have not forgiven him for proving them wrong – and the vindictiveness will continue. It won't stop him; his genuine strengths of thought, action and toughness will get through and help to win victory."

Kinnock insisted that Miliband was a "real leader" who was seeking to change Britain in far-reaching ways and that his authenticity would impress voters in the end. "Ed has great personal and political courage – the way in which he's taken on [Rupert] Murdoch, big energy, banks, and effectively prevented our military engagement in Syria – just some examples – shows that. He's got high intelligence and articulates deep concerns."

Kinnock's comments followed a stark warning from former home secretary David Blunkett, who said a Tory victory in 2015 would result in a government focused on "excluding the Labour party" and keeping it out of office "for as much as 15 years". Blunkett warned of the impact of future boundary and funding changes, as he announced his intention to stand down as an MP next year.

Miliband has faced a difficult period since May's local and European elections, in which Labour failed to make the breakthroughs expected of an opposition party a year before a general election. Mandelson said last week that Miliband's problems go further than PR gaffes and suggested he had failed to develop and promote a "convincing and vivid narrative" of what he and his party were offering voters.

Signs that other grandees are rallying follow calls from members of the 2010 intake for the party to stop its infighting, cast off self-doubt and get behind their leader. His aides said they were incensed by recent media coverage that wrongly made out that Labour was in crisis while playing down Tory problems, including David Cameron's impending defeat over his attempts to block Jean-Claude Juncker from becoming the next president of the European Commission.

They said that in the coming weeks there would be more policy reports on economic growth and devolution that would show the party was full of ideas and ready to fight on a radical platform.

Labour's opinion poll lead over the Tories – which seemed to have all but disappeared a few weeks ago – is back up to four points in the latest Opinium/Observer survey. But just 23% of the electorate say that they approve of the way Miliband is handling his job, compared with 49% who disapprove. Cameron fares better, with 37% approving of his performance against 47% who disapprove. Even among Labour's own voters only just over half say they approve of Miliband's performance. The poll puts Labour on 35%, the Tories on 31%, Ukip on 17% and the Lib Dems on 7%.

As part of their fightback, Labour strategists say they will seek to drive home the message that while Cameron is the creation of his spin and believes in little, Ed Miliband is authentic, consistent and brave, if not a polished PR product.

Diane Abbott, the MP who Miliband sacked from the shadow cabinet last year, also offered her support to Miliband yesterday, saying he had kept the party together following its crushing 2010 defeat. "There is no question in my mind that Labour selected the right Miliband for leader," she said. "I think he has done a very good job holding the party together and has produced some excellent policies with a great deal of attraction to the electorate."

One former minister was reported at the weekend as saying that Miliband had once been thought of as a reforming leader in the mould of Kinnock but that "people are increasingly thinking he is Iain Duncan Smith".