The 57-year-old presenter grilled the Shadow Chancellor over Labour's plans for the UK economy, asking him to clarify the numbers that have tripped up Jeremy Corbyn and Diane Abbott previously.
McDonnell started: "We're the only political party that I promise will identify the policy and the cost.
"The only figures in the Tory manifesto are the page numbers," he joked.
The Shadow Chancellor remained confident about Labour's chances next week
Eamonn grilled John about Labour's economic policies
We're the only political party that I promise will identify the policy and the cost
The Labour hopeful continued to say that he has a good working relationship with the business representative organisations, adding: "I've been working with big business organisations since I became Shadow Chancellor two years ago.
"We've worked incredibly closely with representative organisations and have incorporated their views. We have a good working relationship with them," he continued.
However, Holmes swiped at McDonald: "You might be a nice guy but if you bankrupt the country it might be another thing."
GMB: Piers Morgan & Susanna Reid take on politicians Tue, January 24, 2017
Good Morning Britain hosts Piers Morgan and Susanna Reid talk politics with Theresa May, Nigel Farage, Donald Trump and more.
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Good Morning Britain talks politics with Theresa May, Donald Trump, Nigel Farage, & more
McDonnell didn't appear to be in the mood for joking, quickly replying: "I'm trying to raise the level of economic debate around the country and I think increasingly now, the Tories are insulting people's intelligence with silly slogans. "
When asked if he thought Labour could romp to victory next week, the could-be Chancellor was optimistic: "I'm convinced we're going to win. I think we'll narrow the gap because people want debate policies."