He said the Labour leadership is facing a “360-degree struggle to survive” with critics “coming at us” on a daily basis.
Speaking to the Guardian, he directed his fire at “oligarchs protecting their power base”, but also said some left-leaning sections of the press were not giving the Labour leader a “fair ride”.
A key ally of Corbyn, he recently warned a “soft coup” was under way against his leadership of the party.
After defeat in the Copeland by-election brought the party to the “cliff-edge”, McDonnell said it was time to unite around the leader in the face of the media criticism.
“Jeremy Corbyn is trying to transform our society so that it is radically more equal, radically more fair, radically more democratic,” he said.
“The whole media establishment (is) owned by people whose power is entrenched.
“They are trying to destroy a socialist who is trying to transfer power from the establishment to the people. That is their job to do. The oligarchs are protecting their power base.”
McDonnell added that the Guardian had become “part of the New Labour establishment”.
“As a result of that, you feel dispossessed because your people are no longer in power,” he said.
On Friday, Corbyn was warned by senior Labour figures that the “tide was turning” against the leadership, following reports that the party’s membership was losing members by the tens of thousands.
McDonnell told the Guardian the figures were “mostly seasonal or the result of the lapsing of members” after The Times said around 26,000 members had quit since last summer, according to leaked data.
The shadow chancellor and shadow home secretary Diane Abbott, two of Corbyn’s closest allies, have insisted the party is still the largest in Europe.
However Labour MP John Mann said the scale of the membership fall-off was a “concern” and warned the leader he needed to “turn it round pretty quickly”.
“This is more than the natural drop-off you would expect to see,” he said.
Corbyn is expected to address a rally of NHS workers in London accompanied by McDonnell and Unite leader Len McCluskey on Saturday.