"It's not a foregone conclusion, a lot of Labour's policies are very popular and when you contrast it with what the Tories are offering today in their manifesto, there's a genuine contrast and a real choice for voters", he said.
The statement follows Tuesday's remarks that a seat total of 200 - almost 30 fewer than the number won under Ed Miliband in 2015 - would be a "successful" result for Corbyn.
Chief Secretary to the Treasury David Gauke called the manifesto a "shambles" and said the plans weren't worth the risk to the economy.
"It will lead us through Brexit while putting the preservation of jobs first", he said of the manifesto, appearing in front of Labour's election slogan: "For the many, not the few".
And yet - bizarrely - while the debate was going on, the Labour leader posted another tweet, saying: "Theresa May, why not debate me?"
United Kingdom opposition Labour Party leader Jeremy Corbyn will on Tuesday unveil a "radical and responsible" plan for government vowing it will change the country and govern "for the many not the few", the media reported.
The BBK don sat down with the Labour party politician to discuss ways to engage young people with politics, following some Twitter interaction.
Labour's election manifesto launch was overshadowed by fresh questions over how it would pay for a string ambitious promises costing billions of pounds. We are getting constant feedback from our members and there's been an incredible response, a very positive response.
She told BBC Radio Scotland: "I hope he is wrong, I want to see a Labour government across the whole of the United Kingdom, that's what I'm spending every waking moment of my day campaigning for".
"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".
By moving to the left, Labour has cleared the way for May to put her stamp on the center ground of British politics and appeal for traditional supporters of the opposition party who backed leaving the European Union.
The governing Conservatives say Labour has an outdated economic vision that will stifle growth.
It came as Labour made further gains in the polls, as support for the party reached its highest point of the general election campaign so far. McCluskey's initial prediction of 200 seats for Labour would suggest a Tory majority in the Commons of about 80.