The Unite leader suggested winning 200 seats - almost 30 fewer than in 2015 - would be a "successful" result for United Kingdom leader Jeremy Corbyn.
I'm very proud of our manifesto for the many not the few.
The 128-page document dedicates just one page to the Private Rented Sector, alongside pages for home ownership and council and social tenants.
Critics say the move leftwards stirs memories of the party's 1983 manifesto, described then by a Labour lawmaker as "the longest suicide note in history" for helping the Conservatives, and some have questioned how the party can fund its program.
"Jeremy Corbyn can't deliver any of this", a Tory spokesman said.More news: Oil producers' talk of longer cuts spurs optimism
It is therefore proposing to introduce new legal minimum standards to ensure properties are "fit for human habitation", giving tenants the right to take action if their rental property is sub-standard.
Len McCluskey, the general secretary of Unite, said it would be "extraordinary" if Labour won on June 8, given Corbyn's public image and the criticism of the party in the media.
The manifesto still has to be approved by around 80 Labour figures, including the shadow cabinet and the party's National Executive Committee, reports the BBC.
Last month, Theresa May said she would hold an early parliamentary election at the beginning of June in order to overcome divisions in the parliament and strengthen her hand in talks with the European Union on Brexit. People like me are always optimistic ... things can happen. I'm full optimism for what Labour can achieve. "So I believe in these next few weeks, we can do it".
Ms Dugdale, the party's leader in Scotland, said she defined success as "winning the election".More news: U.S. envoy vows to call out countries backing North Korea
Mr McCluskey claimed he was not setting a benchmark on which his own support for Mr Corbyn rested.
Mr McCluskey said it was hard for the party to challenge now only two years after losing an election and one year after Labour MPs attempted to replace Mr Corbyn as leader, a move he said gave the impression of a divided party in the minds of many voters.
But the union leader did not rule out victory entirely, adding that Labour was "fighting for every vote".
"Labour's policies will make Britain a better and more equal society so we're trying to pump out that message".More news: House, Senate Russia investigators promise to plow ahead
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