Shadow chancellor John McDonnell has rejected suggestions that Labour MPs are distancing themselves from Jeremy Corbyn as he challenged a journalist to reveal their sources.
She will say these voters are a "majority for change" and are not served by either the SNP or the Conservatives.
Britain's opposition Labour Party launched a "radical and responsible" election manifesto on Tuesday, showing a shift to the left to try to capitalize on voters' concerns over education and health before next month's vote.
A draft version of the Labour manifesto was leaked last week.
Noting how the Prime Minister was hiding from a live head-to-head, he said: "So come on Mr Hammond, come and have a debate with me because once we've had that debate people will realise just what your government is all about: more austerity stifling our economy; failing to ensure that people in work are properly paid and undermining those people who can't work as a result of your benefit cuts".
"He's got now just under four weeks to try to see if you can break through that image and it's going to be a very, very hard task.whether that breakthrough can happen, we'll wait and see".
Mr McCluskey's prediction of just 200 seats for Labour and a big win for Theresa May and the Conservatives will be interpreted in two ways inside the Labour Party and by political opponents.More news: Turkey's Erdogan and Trump: Kindred spirits, divergent agendas
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And, in a comment that will infuriate many Labour activists and MPs, Mr McCluskey said: "I believe that if Labour can hold on to 200 seats or so it will be a successful campaign".
"Whether that breakthrough can happen, we'll wait and see".
Both Gordon Brown and Ed Miliband resigned after leading Labour to defeats in 2010 and 2015 but, amid speculation that jockeying for position has already begun, there have been suggestions that Mr Corbyn could stay on if he equals the 30.4% vote share that Ed Miliband got in 2015.
It also argues that Labour's plans to borrow to invest in the country's infrastructure - with a national transformation fund investing £250 billion over 10 years - should be included in the overall borrowing figures.
An analysis by the party of energy company accounts in 2013 found they were paying out £3.2 billion a year in dividends and interest payments.
"It's ordinary working people who will pay for the chaos of Corbyn".
It insists they are fully costed and will be fully covered by tax rises, including increasing corporation tax from 19% to 26% and raising the top rate of tax for earnings above £80,000 to 45p and to 50p for income over £123,000.