He blamed what he described as media bias against Corbyn, a veteran left-winger who after decades on the rebellious fringe of the party unexpectedly became Labour leader in 2015 on a wave of enthusiasm from grassroots party members.
If Labour secured 200 seats it would be their worst result since 1935. I'm full optimism for what Labour can achieve.
Many local Labour politicians said "success" was winning a majority of the 650 seats available in parliament and warned that McCluskey's comments were demoralising.
Kezia Dugdale has vowed to carry on as Scottish Labour leader even if her party suffers a complete wipe-out at next month's General Election.
Meanwhile, elsewhere on the campaign trail, Labour's Kezia Dugdale said the vote on June 8 could determine if the United Kingdom continues to exist at all.
At the launch, Mr Corbyn said "clearly we are not going to freeze benefits" and the party later insisted that commitment was covered by its pledge of an extra £2bn for universal credit.
Scottish Labour's ruling executive has rejected proposals for a coalition with the Conservatives to run West Lothian Council.
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Mentions of either Theresa May or Jeremy Corbyn by the Conservatives increased from 75% to more than four out of every five posts (84%) across Facebook and Twitter.
On the chances of a Labour victory, Mr McCluskey said: "In terms of the imagery of Jeremy, that's a huge task".
"I think there's a rumbling, a subterranean rumbling at the moment, where people are saying we want change".
It said it could cut annual water bills by £100 and energy bills by £120, stopping dividends to shareholders while cutting the cost of interest payments on the companies' debts as the government would be able to borrow more cheaply.
"But while his figures are a fantasy, it's ordinary working families who will pay with higher taxes or higher bills". In the main, that contribution will come from dropping the threshold for the additional rate of income tax (45%) from £150,000 to £80,000 while those with earnings over (a very precise) £123,000 paying tax at a [re-introduced] 50% rate.
"Jeremy Corbyn can't deliver any of this", a Tory spokesman said.
Labour has also said it would return the railways to public ownership as franchises expire, or in some cases using franchise reviews or break clauses.