One of the most significant differences between Keir Starmer and Jeremy Corbyn is the Parliamentary Labour Party. A great many more MPs supported Starmer’s leadership campaign than backed Corbyn in either of his, and the overwhelming majority of them are now reconciled to its success. The PLP is also markedly smaller than it was for most of Corbyn’s leadership, Labour having suffered a net loss of 59 seats at the end of last year.

For Starmer, those differences add up to two practical strengths. The first is that a bigger proportion of the PLP are in frontbench roles, and thus subject to collective responsibility. The second is that the scale of his support and the power of his patronage means he can impose his authority on Labour MPs in a way that Corbyn, who at no point had an entirely loyal Shadow Cabinet, never really could.

The Liberal Democrats have discovered this to their cost. Winning any publicity as the fourth-largest party in the Commons is a difficult ask at the best of times. In a majority parliament that has resolved Brexit, it is more difficult still – pandemic or not. Since their 2015 wipeout, getting anything done as a Liberal Democrat has meant working cross-party.

That is especially true now. An impending leadership election increases the incentive for those with ambition to reach out across the aisle in search of support for whatever cause they happen to be agitating for. But while Labour MPs were a ready source of troops for cross-party work in the last Parliament, Liberal Democrats are now finding their advances rebuffed. MPs seeking signatures on letters are told they will not be forthcoming.

Why? Liberal Democrats spin it as a diktat imposed by the new Labour leadership. “It limits the freedom of MPs,” says one. “It feels like the new leadership want to get on top of the PLP, and [is] encouraging these kind of things to go central.” For those that have only known the relative free-for-all of 2015 to 2020, that is indeed what it smells like.

The reality is more prosaic, as Labour MPs with longer memories attest: convention is reasserting itself, or rather, Starmer has the power to assert it. This is what Labour leaders can do when they are on top of the PLP. Opposition frontbench teams were always supposed to lead on issues collectively, rather than allowing MPs to make the running with letters that, as one shadow minister puts it, “proliferate and everyone feels obliged to sign, as constituents ask whether you will”.

What makes this Parliament different for Labour MPs is that there are no longer competing sources of political legitimacy in the eyes of their colleagues or, crucially, the media: Starmer is the only show in town. If they want their leadership to adopt a policy position now, they have two options – lobby the relevant frontbencher to adopt it, or else be seen to pick a fight with the leadership publicly, as the Socialist Campaign Group is willing to do. Unfortunately for Liberal Democrat MPs in need of ballast, the vast majority are picking the first option.