it does not matter who supports his arguments.
The TV coverage/highlights will be of the exchanges between Corbyn and Cameron. Some old duffer, who quite likes Putin, middle eastern terrorists and Gerry Adams will get torn to pieces by Cameron. Imagine the carnage if Boris enters the game. It will be humiliating.
the average joe will see that he is incapable of leading his party in the commons, let alone the country. (much like Ed)
Image plays a part these days (like it or not)...he best get ready for severe scrutiny (which he has never ever had) and pictures of him smiling, chatting to some seriously dodgy people will be online for the average voter to see.
He is already having to explain himself about supporting/giving money to a holocaust denier...this is just the start
also, the nation is not going to vote for some old duffle-coat wearing codger to lead us on the world stage.