"But our votes are being hijacked by Tory and SNP politicians who want to use how we voted in referendums and general elections to assume something about the kind of future we want to see".
F our in 10 Labour backers said they may switch parties before polling day, with a third of those to the Conservatives.
In a candid interview, Mr McCluskey said: "The scale of the task is huge".
In a sign of the pressures within the Labour camp as it continues to trail in the polls, he told the Politico website: "I don't see Labour winning". "We've got half a million members, the biggest political party in Europe, out there campaigning and Labour MPs right the way across the political spectrum in the Parliamentary Labour Party working together for a Labour government", he insisted.
"In particular Labour will ensure there is no return to a hard border between Northern Ireland and the Republic of Ireland and that there is no change in the status or sovereignty of Gibraltar", the partY said in the manifesto.More news: USA 'democracy is at risk' - lawmaker wants Trump impeached
The Scottish Labour leader called on voters to "reject the extremes of both the SNP and the Tories" and instead to back her party to help bring about "progressive change" in the country.
When Parliament was dissolved at the end of April, Labour held 229 seats compared to the Tories' 330.
The latest opinion polls have Labour around 30% or more with the Conservatives above 40%.
"Vote for Jeremy Corbyn. show us how it's done!"
On the chances of a Labour victory, Mr McCluskey said: "In terms of the imagery of Jeremy, that's a huge task". You've got the guy.More news: Brazilian President Michel Temer, accused of bribery, says he will not resign
"We can not allow this to be the choice at this election - because it doesn't represent what the vast majority of people in this country want".
Scottish Labour believes that together we're stronger.
Mr Gething admitted he had not backed Mr Corbyn in the last two Labour leadership contests, but accepted the party had voted "decisively" for him to be its leader and candidate for prime minister.
However, the Unite leader did not rule out victory completely, saying Labour was "fighting for every vote" and praising Mr Corbyn's performance in the campaign.
"I don't think it matters whether I'm a fan or not - it matters whether I think he can do the job in running the country".More news: Always Dreaming is 4-5 early favorite for Preakness
Mr McCluskey claimed he was not setting a benchmark on which his own support for Mr Corbyn rested.
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