Today’s Times (£) reports the astonishing claim that Chris Fay, a former social worker and Labour councillor who passed information on Leon Brittan to Tom Watson, has admitted that he was “well up for a witch hunt against rightwing Tories.”

This twist of the knife comes after a brutal few days for Labour’s deputy leader, as his high-profile crusade against Brittan, the former home secretary, has blown up in his face.

Watson has done some good work in helping the victims of sexual abusers: he claims in a piece for the Huffington Post to have helped secure the conviction of two paedophiles.

But something, be it a love of the limelight or simple zealotry, seems to have led him to overstep.

Instead of confining himself to passing information to the police he publicly reiterated, in a national newspaper, a series of allegations against Brittan, including some very emotive quotations from alleged victims, which have since fallen apart.

It was also reported on the BBC’s Panorama that he pressured the authorities to re-open their investigation into Brittan after the police closed it for lack of evidence.

There’s a slight whiff of Senator McCarthy about this story now, especially after Panorama interviewed several critics of apparent political interference in police investigations for “a step upwards”.

Watson is starting to fail the smell test, and senior Tories have noticed.

Boris Johnson has attacked the police delay in informing Brittan that the case had been dropped (again), whilst Theresa May has offered coded criticism of Watson’s conduct.

Nigel Evans MP has said that “what he did was repeat allegations which just added fuel to the misery” whilst Dominic Grieve, the former attorney-general, is quoted as saying:

“The more the allegations are surrounded by individuals making public statements about it the more it becomes harder to investigate because facts and fantasies start to become entangled and it’s hard to disentangle them.”

It is already apparent that the Tories are keen to prevent Labour from confining the damage caused by Corbyn to the leader and his circle. We can expect any chink in the armour of the deputy, widely viewed as a check on the far left, to be exploited ruthlessly.

Watson’s fondness for grandstanding risks not only overshadowing or even undermining genuine investigations into abuse (including his own successes), but permanently tarnishing his newly-minted veneer as the tough, straightforward gatekeeper of electable Labour.

The stain of this story could leave him permanently diminished, reducing his ability either to take on the Tories or – more pressingly – the Red Guards fanning out through his own party.