Jeremy Corbyn is taking to the airwaves to make the case against British air strikes in Syria ahead of a potentially dramatic showdown with his shadow cabinet.
The Labour leader, who has spent the last couple of days amassing evidence of grass roots support for his position, is expected to deliver a riposte to critics in a high-profile interview.
The move raises the prospect that the leader could pre-empt what had been billed as a crunch meeting of the shadow cabinet on Monday to decide whether Labour MPs will get a free vote.
Later that evening he will speak to a gathering of the Parliamentary Party.
Mr Corbyn has already been accused of trying to "bounce" his senior colleagues – most of whom are thought to favour extending military action against Islamic State (IS) from Iraq into Syria - into submitting to his view by sending out a survey asking for the views of Labour members.
David Cameron gave his clearest indication yet yesterday that he is committed to holding a vote, saying he hoped MPs would do the right thing "when the choice comes".
Setting out his rationale for action last week, Mr Cameron had made clear he would only bring the matter before the Commons if he was confident of securing a significant majority.
However, Government Whips believe a number of Tory sceptics have now been won over, and Defence Secretary Michael Fallon and Foreign Secretary Philip Hammond have spent the weekend working to convince individual Labour MPs.
There have been suggestions that up to half the Opposition are minded to support the extension of air strikes – although numbers will depend on whether the leadership opts to impose a whip.
Downing Street sources said the motion being drawn up would have the United Nations Security Council Resolution - passed unanimously in the wake of the deadly terrorist attacks on Paris - "front and centre".
It will note that the resolution backs states to take "all necessary measures" to prevent terrorist acts by IS and to "eradicate the safe haven they have established over significant parts of Iraq and Syria".
The text will also underline the importance of a political settlement for the civil war in Syria, and note that UK military capabilities, such as the high-precision Brimstone missile, will help minimise civilian casualties.
If Mr Corbyn, who is appearing on the BBC's Andrew Marr show, attempts to whip his MPs to oppose air strikes he faces the prospect of senior resignations and widespread defiance.
But while his closest ally, shadow chancellor John McDonnell, has expressed a preference for a free vote that would reduce tensions, Mr Corbyn has seemed unwilling to go down that path.