Until recently, Tom Watson was a shrunken figure in every sense. Labour’s deputy leader had become more famous for his weight-shedding diet than for the influence he wielded in his party. He had earned the undying enmity of the Corbynites for attempting to persuade their hero to quit as leader and they schemed to replace him as deputy with someone more pliable. Labour moderates had given up on him for failing to deliver on reassurances that he could fix the party. Whenever he made public criticism of antisemitism or entryism by the far left, a gang of Corbynites in the shadow cabinet would be organised to verbally batter him at the body’s next meeting.

At last autumn’s conference in Liverpool, the deputy leader was such a diminished figure that he was denied a speaking slot from the platform and all he could do was pretend to be happy about it.

Now Tom Watson is back. He is throwing his political weight around, speaking publicly and boldly about what is rotten about his party and doing so with a force and an impact that is infuriating Jeremy Corbyn and his inner circle.

The deputy was instrumental in forcing the suspension of Chris Williamson, a close ally of Mr Corbyn, when video was released of the Derbyshire MP telling a room of applauding activists that Labour had been “too apologetic” about antisemitism. The deputy leader also played a pivotal role in forcing a highly reluctant Mr Corbyn finally to commit to a fresh referendum on Brexit. The deputy’s biggest challenge to the Corbynites has been to propose the formation of a new group within the party to supply structure and give voice to its social democratic and other non-Corbynite traditions.

This resurgence owes an unacknowledged debt to the MPs who have deserted Labour to form the Independent Group. Though relatively small in number, the breakaways are having a significant impact on the power dynamics within the party they departed. The threat of more desertions is unnerving for Mr Corbyn and it is emboldening the moderates who have chosen to stay within the party for the time being.

Labour’s deputy leader is riding a wave of feeling that power is leaking away from its leader. The erosion of Mr Corbyn’s position was inevitable because of the laws of political entropy. All ideological movements surge and then decline as their original appeal is decayed by disappointment. It is not surprising that this is happening to Corbynism. It also happened to Thatcherism and Blairism, although they had the benefit of getting into government before the decline set in.

‘Emily Thornberry demanded of Watson: “Which members of the shadow cabinet do you want to sack?”’ Photograph: Oli Scarff/AFP/Getty Images

Time is the deadliest enemy of every leader and time has deprived Mr Corbyn of one of his most important original selling propositions. He has projected himself as the rebellious insurgent, the anti-establishment underdog. The record as a rebel helped him to win the Labour leadership in the first place. The anti-establishment look helped him to see off the 2016 attempt by Labour MPs to dislodge him. Underdog status helped him to a better than expected result at the 2017 general election.

The passage of time has made it harder for Mr Corbyn to convincingly cast himself as the plucky insurgent, certainly not in terms of the Labour party. The leader and his allies grip all the important institutions of the party. The Corbynites occupy the commanding heights of the party; they have the general secretaryship, they command the national executive committee and they dominate the shadow cabinet. Everything they survey they control. This hegemony comes with a price. As one Labour MP puts it: “They can’t make themselves victims any more. It’s all on them. It’s owned by Jeremy.”

Among the things he owns is the months of deliberate ambiguity and prevarication about Brexit, all intended to avoid having to commit to the people’s vote. He has finally been compelled to (sort of) support giving the people a fresh say, but he is not getting much credit for the switch because everyone can see that it has been made begrudgingly, cynically and insincerely and without any clear commitment that he wants to be a vigorous campaigner to reverse Brexit. Labour MPs report that their leader made the statement with all the enthusiasm of a hostage reading out a ransom demand. This last-gasp and unconvincing conversion may be too late to make any difference anyway if Theresa May manages to squeak a tweaked version of her withdrawal deal through parliament. It looks not like a decision founded in principled conviction, but an act of weakness and desperation designed to placate restive party members and stem further defections.

The damage the breakaways have inflicted on Labour’s poll rating puts an even sharper focus on Mr Corbyn’s dire personal ratings. Imperfect as the polls may be, they offer some guide to the mood of the British public and the results are not encouraging for Labour. Even Mr Corbyn’s most devoted followers have to be concerned that his personal ratings are the worst for any opposition leader in more than 30 years, especially when he is facing a Tory government that has been tearing itself apart. After nearly four years as the party’s leader, Labour’s popularity – or lack of it – is something else that is owned by Mr Corbyn and his comrades.

The party has only been polluted with anti-Jewish racism since Mr Corbyn and his allies were given the keys to the Labour house. Photograph: Victoria Jones/PA

To which we can also add the poison of antisemitism that has entered Labour’s bloodstream on his watch. The party did not have this problem under Ed Miliband, Gordon Brown or Tony Blair. Three very different kinds of Labour leader; none presided over a party hosting the most vile manifestations of antisemitism. The party has only been polluted with anti-Jewish racism since Mr Corbyn and his allies were given the keys to the Labour house and took responsibility for what kind of person is allowed to live there.

This issue reaches beyond and deeper than ideology. Antisemitism goes to core moral codes, people’s sense of who they are. Tom Watson is one of the people who has been discovering who he is. When Luciana Berger declared that she had been driven out of Labour by antisemites, the deputy leader reacted to her testimony by saying: “It was the worst day of shame in the Labour party’s 120 years of history: a pregnant, young MP bullied out of the Labour party by racist thugs.”

The Corbynites are furious with him and their caucus in the shadow cabinet is still trying to shut him up. After he called for a recasting of Labour’s frontbench to include more people from all wings of the party, there was an attempt to intimidate him into silence at the next meeting of the shadow cabinet. Emily Thornberry, the shadow foreign secretary, turned on him by demanding: “Which members of the shadow cabinet do you want to sack?”

This deliberately missed his point, which is that Labour needs to be a pluralistic party, not a sectarian outfit in the control of one narrow faction. I am told, and this is interesting, that John McDonnell, the shadow chancellor, did not join the attempt to gag the deputy leader.

What is the true purpose of his new formation to organise the non-Corbynite wings of the Labour party? Mr Watson says he wants to try to hold them together and within the party. The more conspiracy-minded, and he is a figure who attracts conspiracy theories, see it as an embryonic vehicle for another breakaway from Labour. It could turn out to be either. I suspect he isn’t sure where it will lead in the end. The important point is that he has become sufficiently emboldened to publicly advocate the idea.

It is possible that he will overreach. The Corbynites could still have enough sway over Labour members to punish him by resurrecting their plots to evict him from the deputy leadership. Mr Watson has told friends: “They may try to run me out of town.”

They may, but the changed context would make that a highly risky and destructive enterprise. An attempt to oust him would almost certainly precipitate another, and larger, exodus of Labour MPs.

The resurgence of Tom Watson shows that the balance of terror within the Labour party has shifted. In the 21 months since the last general election, most of Labour’s moderates have been sounding cowed and running scared. Now it is the Corbynites who have reasons to fear what the moderates might do next.

• Andrew Rawnsley is an Observer columnist