Jeremy Corbyn's left hand man will today sett out Labour's five demands for next week's Budget saying it must not be "desperately designed to save the jobs of a weak Prime Minister and her embattled chancellor".
He is seeking for Hammond to "pause and fix" the controversial new benefits system universal credit; provide new funding to lift the public sector pay cap; announce "serious" funding for infrastructure; "properly fund" public services and - in a nod to communities secretary Sajid Javid's request last month - launch a large-scale public house-building programme.
McDonnell will say: "There has to be a genuine and decisive change of course... [Hammond] wants to pretend he cannot invest on the scale needed, yet he has already borrowed more in his first year as chancellor than any of his predecessors in their first year at the Treasury. There is a better way than this. But it needs a complete break with past failures."
Although they might differ on the details, many Conservative MPs would agree that change is needed, and privately some are seeing the Budget as a make-or-break situation for Hammond and May.
One backbencher told City A.M. "how the Budget lands" would determine whether the chancellor would be removed in the widely-anticipated cabinet reshuffle.
"Getting rid of your chancellor is a big statement - you're basically ripping up your economic plan - but that's what it might come to," he said.
Another said: "Next week is critical, and has been since we missed our chance to reset and get refocused at the party conference."